Woodward MRN3 Series User manual

MRN3 – Mains decoupling relay
(September 2008)
Manual MRN3 (Revision New)

Woodward Manual MRN3 GB
2 TD_MRN3_09.08_GB_Rev.New
Woodward Governor Company reserves the right to update any portion of this publication at any time. Information
provided by Woodward Governor Company is believed to be correct and reliable. However, no responsibility is as-
sumed by Woodward Governor Company unless otherwise expressly undertaken.
© Woodward 1994-2008

Manual MRN3 GB Woodward
TD_MRN3_09.08_GB_Rev.New 3
Contents
1.Introduction and application ....................................................................... 5
2.Features and characteristics....................................................................... 6
3.Design ........................................................................................................... 7
3.1Connections ...................................................................................................................... 7
3.1.1Analog input circuits ...................................................................................................... 7
3.1.2Blocking input................................................................................................................ 7
3.1.3Reset input .................................................................................................................... 7
3.1.4Output relays................................................................................................................. 8
3.1.5Fault recorder................................................................................................................ 8
3.2Parameter settings .......................................................................................................... 10
3.3LEDs ............................................................................................................................... 11
3.4Front plate ....................................................................................................................... 12
4.Working principle ....................................................................................... 13
4.1Analog circuits................................................................................................................. 13
4.2Digital circuits .................................................................................................................. 13
4.3Voltage supervision......................................................................................................... 13
4.3.1Selection of star or delta connection ........................................................................... 14
4.4Principle of frequency supervision................................................................................... 15
4.5Measuring of frequency gradient (MRN3-2) .................................................................... 15
4.6Vector surge supervision (MRN3-1)................................................................................ 16
4.6.1Measuring principle of vector surge supervision ......................................................... 17
4.7Voltage threshold value for frequency measuring ........................................................... 21
4.8Blocking function ............................................................................................................. 21
5.Operation and setting ................................................................................ 22
5.1Display ............................................................................................................................ 22
5.2Setting procedure............................................................................................................ 23
5.3System parameter........................................................................................................... 24
5.3.1Display of residual voltage UEas primary quantity (Uprim/Usec) .................................... 24
5.3.2Y – Switch over ........................................................................................................... 24
5.3.3Setting of nominal frequency....................................................................................... 24
5.3.4Display of the activation storage (FLSH/NOFL) .......................................................... 25
5.3.5Parameter switch/external trigger for the fault recorder .............................................. 25
5.4Protection parameters..................................................................................................... 27
5.4.1Parameter setting of over- and under voltage supervision.......................................... 27
5.4.2Number of measuring repetitions (T) for frequency functions ..................................... 27
5.4.3Threshold of frequency supervision ............................................................................ 27
5.4.4Tripping delays for the frequency elements ................................................................ 28
5.4.5Parameter setting of vector surge supervision (MRN3-1) ........................................... 28
5.4.6Parameter setting of frequency gradient (MRN3-2) .................................................... 29
5.4.7Voltage threshold value for frequency and vector surge measuring
(df/dt at MRN3-2) ........................................................................................................ 29
5.4.8Adjustment of the slave address ................................................................................. 29
5.4.9Setting of Baud-rate (applies for Modbus Protocol only)............................................. 29
5.4.10Setting of parity (applies for Modbus Protocol only) ................................................ 29
5.5Adjustment of the fault recorder ...................................................................................... 30
5.5.1Number of the fault recordings .................................................................................... 30
5.5.2Adjustment of trigger occurrences............................................................................... 30
5.5.3Pre-trigger time (Tpre)................................................................................................... 30
5.6Adjustment of the clock ................................................................................................... 30
5.7Additional functions ......................................................................................................... 31
5.7.1Setting procedure for blocking the protection functions............................................... 31
5.8Indication of measuring values........................................................................................ 33
5.8.1Measuring indication ................................................................................................... 33
5.8.2Min./Max.- values ........................................................................................................ 33
5.8.3Unit of the measuring values displayed....................................................................... 34
5.8.4Indication of fault data ................................................................................................. 34
5.9Fault memory .................................................................................................................. 34

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5.9.1Reset ........................................................................................................................... 35
5.9.2Erasure of fault storage ............................................................................................... 35
6.Relay testing and commissioning ............................................................ 36
6.1Power-On ........................................................................................................................ 36
6.2Testing the output relays ................................................................................................. 36
6.3Checking the set values .................................................................................................. 37
6.4Secondary injection test .................................................................................................. 37
6.4.1Test equipment............................................................................................................ 37
6.5Example of test circuit ..................................................................................................... 38
6.5.1Checking the input circuits and measuring functions................................................... 39
6.5.2Checking the operating and resetting values of the over/under voltage functions..... 39
6.5.3Checking the relay operating time of the over/undervoltage functions ........................ 39
6.5.4Checking the operating and resetting values of the over/under frequency functions 40
6.5.5Checking the relay operating time of the over/underfrequency functions.................. 40
6.5.6Checking the vector surge function ............................................................................. 40
6.5.7Checking the external blocking and reset functions .................................................... 41
6.6Primary injection test ....................................................................................................... 42
6.7Maintenance.................................................................................................................... 42
7.Technical data ............................................................................................ 43
7.1Measuring input circuits................................................................................................... 43
7.2Common data.................................................................................................................. 43
7.3Setting ranges and steps................................................................................................. 44
7.3.1Interface parameter ..................................................................................................... 45
7.3.2Parameters for the fault recorder................................................................................. 45
7.4Output relays ................................................................................................................... 45
8.Order form................................................................................................... 46

Manual MRN3 GB Woodward
TD_MRN3_09.08_GB_Rev.New 5
1. Introduction and application
The MRN3 is a universal mains decoupling device and covers the protection requirements from
VDEW and most other utilities for the mains parallel operation of power stations.
•Over/ and under voltage protection,
•over/ and under frequency protection,
•extremely fast decoupling of generator in case of mains failure (MRN3-1) or
•rate of change of frequency df/dt (MRN3-2)
Because of combination of three protection functions in one device the MRN3 is a very compact
mains decoupling device. Compared to the standard used single devices it has a very good
price/performance ratio.
For applications where the single protection functions are required SEG can offer the single MR-
relays as follows:
•MRU3-1 four step independent over-/ and under voltage protection (also used for
generator earth fault protection).
•MRU3-2 two step independent over-/ and under-voltage protection with evaluation of the
symmetrical voltage components.
•MRF3 four step independent over/ and under- frequency protection and two step
frequency gradient supervision df/dt.
•MRG2 generator mains monitor / vector surge detection.

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2. Features and characteristics
•Microprocessor technology with watchdog,
•effective analog low pass filter for suppressing harmonics when measuring frequency and
vector surge,
•digital filtering of the measured values by using discrete Fourier analysis to suppress
•higher harmonics and d.c. components induced by faults or system operations,
•integrated functions for voltage, frequency and vector surge in one device as well as single
voltage, frequency and vector surge devices,
•two parameter sets,
•voltage supervision each with two step under-/and overvoltage detection,
•frequency supervision with three step under-/or over frequency (user setting),
•completely independent time settings for voltage and frequency supervision,
•adjustable voltage threshold value for blocking frequency and vector surge measuring,
•display of all measuring values and setting parameters for normal operation as well as
tripping via a alphanumerical display and LEDs,
•display of measuring values as primary quantities
•Storage of trip values and switching-off time (tCBFP) of 5 fault occurrences (fail-safe of
voltage),
•recording of up to eight fault occurrences with time stamp
•for blocking the individual functions by the external blocking input, parameters can be set
according to requirement,
•user configurable vector surge measurement 1-of-3 or 3-of-3,
•reliable vector surge measuring by exact calculation algorithm,
•suppression of indication after an activation (LED flash),
•free assignment for output relays,
•display of date and time,
•in compliance with VDE 0435, part 303 and IEC 255,
•serial data exchange via RS485 interface possible; alternatively with SEG RS485
Pro-Open Data Protocol or Modbus Protocol.
Important:
For additional common data of all MR-relays please refer to technical description "MR - Digital Mul-
tifunctional Relays".

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3. Design
3.1 Connections
Figure 3.1: Connection diagram MRN3-1 and MRN3-2
3.1.1 Analog input circuits
The analog input voltages are galvanically decoupled by the input transformers of the device, then
filtered and finally fed to the analog digital converter. The measuring circuits can be applied in star
or delta connection (refer to chapter 4.3.1).
3.1.2 Blocking input
The blocking function can be set according to requirement. By applying the auxiliary voltage to
D8/E8, the previously set relay functions are blocked (refer to 4.8 and 5.7.1).
3.1.3 Reset input
Please refer to chapter 5.9.1.

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3.1.4 Output relays
The MRN3 is equipped with 5 output relays. Apart from the relay for self-supervision, all protective
functions can be optionally assigned:
•Relay 1: C1, D1, E1 and C2, D2, E2
•Relay 2: C3, D3, E3 and C4, D4, E4
•Relay 3: C5, D5, E5
•Relay 4: C6, D6, E6
•Relay 5: Signal self-supervision (internal failure of the unit) C7, D7, E7
All trip and alarm relays are working current relays, the relay for self supervision is an idle current
relay.
3.1.5 Fault recorder
The MRN3 has a fault value recorder which records the measured analog values as instantaneous
values.
The instantaneous values
UL1; UL2; UL3 for star connection
or U12; U23; U21 for delta connection
are scanned at a raster of 1.25 ms (at 50 Hz) and 1.041 ms (at 60 Hz) and saved in a cyclic buffer.
It is possible to store 1 - 8 fault occurrences with a total recording time of 16 s (with 50 Hz) and
13.33 s (with 60 Hz) per channel.
Storage division
Independent of the recording time, the entire storage capacity can be divided into several cases of
disturbance with a shorter recording time each. In addition, the deletion behavior of the fault
recorder can be influenced.
No writing over
If 2, 4 or 8 recordings are chosen, the complete memory is divided into the relevant number of
partial segments. If this max. number of fault event has been exceeded, the fault recorder block
any further recordings in order to prevent that the stored data are written over. After the data have
been read and deleted, the recorder to ready again for further action.
Writing over
If 1, 3 or 7 recordings are chosen, the relevant number of partial segments is reserved in the com-
plete memory. If the memory is full, a new recording will always write over the oldest one.

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TD_MRN3_09.08_GB_Rev.New 9
The memory part of the fault recorder is designed as circulating storage. In this example 7 fault re-
cords can be stored (written over).
Figure 3.2: Division of the memory into 8 segments, for example
Memory space 6 to 4 is occupied.
Memory space 5 is currently being written in
Since memory spaces 6, 7 and 8 are occupied, this example shows that the memory has been as-
signed more than eight recordings. This means that No. 6 is the oldest fault recording and No. 4
the most recent one.
Figure 3.3: Basic set-up of the fault recorder
Each memory segment has a specified storage time which permits setting of a time prior to the
trigger event.
Via the interface RS485 the data can be read and processed by means of a PC (HTL/PL-Soft4).
The data is graphically edited and displayed. Binary tracks are recorded as well, e.g. activation and
trip.

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3.2 Parameter settings
System parameters
Parameter settings MRN3-1 MRN3-2
U
p
rim/Usek X X
Δ/Y X X
fN X X
P2/FR X X
LED-Flash X X
Table 3.1: System parameters
Protection parameters
Setting
parameter
MRN3-1 MRN3-2
U< X X
tU< X X
U<< X X
tU<< X X
U> X X
tU> X X
U>> X X
tU>> X X
T X X
f1X X
tf1 X X
f2X X
tf2 X X
f3X X
tf3 X X
df X
dt X
1/3 X
ΔΘ X
UB< X X
RS485/Slave X X
Baud-Rate* X X
Parity-Check* X X
Table 3.2: Protection parameters *only Modbus
Blocking functions
Parameter settings MRN3-1 MRN3-2
U< X X
U<< X X
U> X X
U>> X X
f1 X X
f2 X X
f3 X X
ΔθX
df/dt X
Table 3.3: Blocking functions

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Parameters for the fault recorder
Parameter setting MRN3-1 MRN3-2
Number of fault
events
X X
Trigger events X X
Pre-Triggerzeit T
p
re X X
Table 3.4: Parameters for the fault recorder
Additional functions
Parameter settings MRN3-1 MRN3-2
Ralay assignment X X
Fault recorder X X
Table 3.5: Additional functions
Date and time
Parameter settings MRN3-1 MRN3-2
Year Y = 99 X X
Month M = 03 X X
Day D = 16 X X
hour h = 07 X X
minute m = 29 X X
second s = 56 X X
Table 3.6: Date and time
The window for parameter setting is located behind the measured value display. The parameter
window can be accessed via the <SELECT/RESET> key.
3.3 LEDs
All LEDs (except LED RS, min. and max.) are two colored. The LEDs on the left side, next to the
alphanumerical display light up green during measuring and red after tripping.
The LEDs below the push button <SELECT/RESET> are lit green during setting and inquiry proce-
dure of the setting values which are printed on the left side next to the LEDs. The LEDs will light up
red after parameterizing of the setting values next to their right side.
The LED marked with letters RS lights up during setting of the slave address of the device for serial
data communication.
The LED marked with the letters FR is alight while the fault recorder is being adjusted.

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3.4 Front plate
Figure 3.4: Front plate MRN3-1
Figure 3.5: Front plate MRN3-2

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4. Working principle
4.1 Analog circuits
The input voltages are galvanically insulated by the input transformers. The noise signals caused
by inductive and capacitive coupling are suppressed by an analog R-C filter circuit.
The analog voltage signals are fed to the A/D-converter of the microprocessor and transformed to
digital signals through Sample- and Hold- circuits. The analog signals are sampled with a sampling
frequency of 16 x fN, namely, a sampling rate of 1.25 ms for every measuring quantity, at 50 Hz.
4.2 Digital circuits
The essential part of the MRN3 relay is a powerful microcontroller. All of the operations, from the
analog digital conversion to the relay trip decision, are carried out by the microcontroller digitally.
The relay program is located in an EPROM (Electrically-Programmable-Read-Only-Memory). With
this program the CPU of the microcontroller calculates the three phase voltage in order to detect a
possible fault situation in the protected object.
For the calculation of the voltage value an efficient digital filter based on the Fourier Transformation
(DFFT - Discrete Fast Fourier Transformation) is applied to suppress high frequency harmonics
and d.c. components caused by fault-induced transients or other system disturbances.
The micro-processor continuously compares the measured values with the preset thresholds
stored in the parameter memory (EEPROM). If a fault occurs an alarm is given and after the set
tripping delay has elapsed, the corresponding trip relay is activated.
The relay setting values for all parameters are stored in a parameter memory (EEPROM –
Electrically Erasable Programmable Read Only Memory), so that the actual relay settings cannot
be lost, even if the power supply is interrupted.
The microprocessor is supervised by a built-in "watchdog" timer. In case of a failure the watchdog
timer resets the microprocessor and gives an alarm signal via the output relay "self supervision".
4.3 Voltage supervision
The voltage element of MRN3 has the application in protection of generators, consumers and other
electrical equipment against over-/and under voltage.
The relay is equipped with a two step independent three-phase overvoltage (U>, U>>) and
under voltage (U<, U<<) function with completely separate time and voltage settings.
In delta connection the phase-to-phase voltages and in star connection the phase-to-neutral
voltages are continuously compared with the preset thresholds.
For the overvoltage supervision the highest, for the under voltage supervision of the lowest voltage
of the three phases are decisive for energizing.

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4.3.1 Selection of star or delta connection
All connections of the input voltage transformers are led to screw terminals. The nominal voltage of
the device is equal to the nominal voltage of the input transformers. Dependent on the application
the input transformers can be connected in either delta or star. The connection for the phase-to-
phase voltage is the delta connection. In star connection the measuring voltage is reduced by 1/ .
During parameter setting the connection configuration either Y or Δhas to be adjusted.
Figure 4.1: Input v.t.s in delta connection ()
Figure 4.2: Input v.t.s in star connection (Y)

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TD_MRN3_09.08_GB_Rev.New 15
4.4 Principle of frequency supervision
The frequency element of MRN3 protects electrical generators, consumers or electrical operating
equipment in general against over- or under frequency.
The relay has independent three frequency elements f1- f3with a free choice of parameters, with
separate adjustable pickup values and delay times.
The measuring principle of the frequency supervision is based in general on the time measurement
of complete cycles, whereby a new measurement is started at each voltage zero passage. The in-
fluence of harmonics on the measuring result is thus minimized.
Figure 4.3: Determination of cycle duration by means of zero passages.
In order to avoid false tripping during occurrence of interference voltages and phase shifts the relay
works with an adjustable measuring repetition. (refer to chapter 5.4.2)
Frequency tripping is sometimes not desired by low measured voltages which for instance occur
during alternator acceleration. All frequency supervision functions can be blocked with the aid of an
adjustable voltage threshold UB in case the measured voltages value is below this value.
4.5 Measuring of frequency gradient (MRN3-2)
Electrical generators running in parallel with the mains, e.g. industrial internal power supply plants,
should be separated from the mains when failure in the intra system occurs for the following rea-
sons:
•It must be prevented that the electrical generators are damaged when mains voltage
recovering asynchronous, e.g. after a short interruption.
•The industrial internal power supply must be maintained.
A reliable criterion of detecting mains failure is the measurement of the rate of change of frequency
df/dt. Precondition for this is a load flow via the mains coupling point. At mains failure the load flow
changing then spontaneously leads to an increasing or decreasing frequency. At active power defi-
cit of the internal power station a linear drop of the frequency occurs and a linear increase occurs
at power excess. Typical frequency gradients during application of "mains decoupling" are in the
range of 0.5 Hz/s up to over 2 Hz/s. The MRN3 detects the instantaneous frequency gradient df/dt
of each mains voltage period in an interval of one half period each. Through multiple evaluation of
the frequency gradient in sequence the continuity of the directional change (sign of the frequency
gradient) is determined. Because of this special measuring procedure a high safety in tripping and
thus a high stability against transient processes, e.g. switching procedure are reached. The total
switching off time at mains failure is between 60 ms and 80 ms depending on the set-ting.

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4.6 Vector surge supervision (MRN3-1)
The vector surge supervision protects synchronous generators in mains parallel operation due to
very fast decoupling in case of mains failure. Very dangerous are mains auto reclosings for syn-
chronous generators. The mains voltage returning after 300 ms can hit the generator in asynchron-
ous position. A very fast decoupling is also necessary in case of long time mains failures. Generally
there are two different applications:
a) Only mains parallel operation no single operation: In this application the vector
surge supervision protects the generator by tripping the generator circuit breaker
in case of mains failure.
b) Mains parallel operation and single operation: For this application the vector surge
supervision trips the mains circuit breaker. Here it is insured that the gen.-set is not
blocked when it is required as the emergency set.
A very fast decoupling in case of mains failures for synchronous generators is known as very diffi-
cult. Voltage supervision units cannot be used because the synchronous alternator as well as the
consumer impedance support the decreasing voltage.
For this the mains voltage drops only after some
100 ms below the pickup threshold of voltage supervision relays and therefore a safe detection of
mains auto reclosing is not possible with this kind of relay.
Frequency relays are partial unsuitable because only a highly loaded generator decreases its
speed within 100 ms. Current relays detect a fault only when short-circuit type currents exist, but
cannot avoid their development. Power relays are able to pickup within 200 ms, but they cannot
prevent power to rise to short-circuit values too. Since power changes are also caused by sudden
loaded alternators, the use of power relays can be problematic.
Whereas the MRN3-1 detects mains failures within 60 ms without the restrictions described above
because they are specially designed for applications where very fast decoupling from the mains is
required.
Adding the operating time of a circuit breaker or contactor, the total disconnection time remains be-
low 150 ms. Basic requirement for tripping of the generator/mains monitor is a change in load of
more than 15 - 20% of the rated load. Slow changes of the system frequency, for instance at regu-
lating processes (adjustment of speed regulator) do not cause the relay to trip.
Trippings can also be caused by short-circuits within the grid, because a voltage vector surge
higher than the preset value can occur. The magnitude of the voltage vector surge depends on the
distance between the short-circuit and the generator. This function is also of advantage to the
Power Utility Company because the mains short-circuit capacity and consequently the energy
feeding the short-circuit is limited.
To prevent a possible false tripping the vector surge measuring can be blocked at a set low input
voltage (refer to 5.4.7). The under voltage lockout acts faster than the vector surge measurement.
Vector surge tripping is blocked by a phase loss so that a VT fault (e.g. faulty VTs fuse) does not
cause false tripping. When switching on the aux. voltage or measuring voltage , the vector surge
supervision is blocked for 5 s (refer to chapter 4.8).
Note:
In order to avoid any adverse interference voltage effects, for instance from contactors or relays,
which may cause over functions, MRN3-1 should be connected separately to the busbar.

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TD_MRN3_09.08_GB_Rev.New 17
4.6.1 Measuring principle of vector surge supervision
When a synchronous generator is loaded, a rotor displacement angle is build between the terminal
voltage (mains voltage U1) and the synchronous internal voltage (Up). Therefore a voltage is
difference ΔU is built between Up and U1 (Fig. 4.4).
Figure 4.4: Equivalent circuit at synchronous generator in parallel with the mains
Figure 4.5: Voltage vectors at mains parallel operation

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The rotor displacement angle ϑbetween stator and rotor is depending of the mechanical moving
torque of the generator shaft. The mechanical shaft power is balanced with the electrical fielded
mains power, and therefore the synchronous speed keeps constant (Fig. 4.5).
Figure 4.6: Voltage vectors at mains failure
In case of mains failure or auto reclosing the generator suddenly feeds a very high consumer load.
The rotor displacement angle is decreased repeatedly and the voltage vector U1 changes its
direction (U1') (Fig. 4.6 and 4.7).
Figure 4.7: Equivalent circuit at mains failure

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Figure 4.8: Voltage vector surge
As shown in the voltage/time diagram the instantaneous value of the voltage jumps to another val-
ue and the phase position changes. This is named phase or vector surge.
The MRN3-1 measures the cycle duration. A new measuring is started at each voltage zero pas-
sage. The measured cycle duration is internally compared with a quartz stable reference time and
from this the deviation of the cycle duration of the voltage signal is ascertained. In case of a vector
surge as shown in fig. 4.8, the zero passage occurs either earlier or later. The established deviation
of the cycle duration is in compliance with the vector surge angle.
If the vector surge angle exceeds the set value, the relay trips immediately.
Tripping of the vector surge is blocked in case of loss of one or more phases of the measuring
voltage.
Tripping logic for vector surge measurement:
The vector surge function of the MRN3-1 super-vises vector surges in all three phases at the same
time. Tripping of the relay can be adjusted for an one phase vector surge (more sensitive measure-
ment). For this the parameter 1/3 has to be set to "1Ph". When the parameter 1/3 is set to "3Ph",
tripping of the vector surge element occurs only if the vector surge angle exceeds the set value in
all three phases at the same time.

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Application hint
Although the vector surge relay guarantees very fast and reliable detection of mains failures under
nearly all operational conditions of mains parallel running alternators, the following borderline cases
have to be considered accordingly:
a) None or only insignificant change of power flow at the utility connection point during
mains failures.
This can occur during peak lopping operation or in CHP stations (Combined Heat and Power)
where the power flow between power station and the public grid may be very low. For detection of
a vector surge at parallel running alternators, the load change must be at least 15 - 20% of the
rated power. If the active load at the utility connection point is regulated to a minimal value and a
high resistance mains failure occurs, then there are no vector surge nor power and frequency
changes and the mains failure is not detected.
This can only happen if the public grid is disconnected near the power station and so the alterna-
tors are not additionally loaded by any consumers. At distant mains failures the synchronous alter-
nators are abruptly loaded by remaining consumers which leads directly to a vector surge and so
mains failure detection is guaranteed.
If such a situation occurs the following has to be taken into account:
In case of an undetected mains failure, i.e. with the mains coupling C.B. closed, the vector surge
relay reacts upon the first load change causing a vector surge and trips the mains C.B.
For detecting high resistance mains failures a minimum current relay with an adjustable trip delay
can be used. A trip delay is needed to allow regulating actions where the current may reach "zero"
at the utility connection point. At high resistance mains failures, the mains coupling C.B. is tripped
by the minimum current relay after the time delay.
To prevent asynchronous switching on, an automatic reclosing of the public grid should be not
possible during this time delay.
A further measure could be, that the load regulation at the utility connection point guarantees a
minimum power flow of 15 - 20% of rated power.
b) Short circuit type loading of the alternators at distant mains failures
At any distant mains failure, the remaining consumers cause sudden short circuit type loading of
the power station generators. The vector surge relay detects the mains failure in about 60 ms and
switches off the mains coupling C.B. The total switch off time is about 100 - 150 ms. If the gen-
erators are provided with an extremely fast short circuit protection e.g. able to detect di/dt, the alter-
nators might be switched off unselectively by the generator C.B., which is not desirable because
the power supply for the station is endangered and later on synchronized changeover to the mains
is only possible after manual reset of the overcorrect protection.
To avoid such a situation, the alternator C.B.s must have a delayed short circuit protection. The
time delay must be long enough so that mains decoupling by the vector surge relay is guaranteed.