Because there is an internal connection between teslameter circuit common and the probe connector case, when the
probe connector is engaged, and the retaining screws tightened the teslameter circuit common will be connected to
the case. Do not make an additional connection between circuit common and the case at any point. Such additional
connection will form a ground loop and may introduce errors in the measured field value.
The shielding provided with the above arrangement should be sufficient protection against EMI in most cases,
especially when the probe cable is shielded. Sometimes it may be found helpful to ground the teslameter case to a
good electrical ground point. Connection can be made to the case by inserting an appropriate lug or terminal under
the head of one of the rear panel fixing screws.
INSTALLATION TECHNIQUES FOR ELECTRICALLY NOISY ENVIRONMENTS
The DTM-152 is a precision electronic measuring device. Because of the nature of the measurements it is asked to do,
it is frequently exposed to conditions that are considerably worse than are normally encountered by precision
instruments. Therefore, the teslameter has been carefully engineered to be as immune as possible to sparks and other
forms of interference through the use of several kinds of power input filtering and a special high-isolation switchmode
power module built into its circuitry. The design has been verified by extensive testing, using high energy sparking in
close proximity to both the teslameter instrument case and the probe. Nevertheless, due care should
always be taken when installing the teslameter system.
The teslameter and its probe must be protected from any chance of receiving a direct hit by a high voltage discharge.
The probe should have shielded cable if the meter is to be used in an electrically noisy environment. The cable shield
is an RFI screen, not a high
current path, so if there is any possibility of an arcing discharge hitting the probe area, then the probe head and part or
all of the cable must be enclosed in a metal tube (nonmagnetic near the probe head) or shielded in some other way.
The probe cable should be routed away from any power, high current or high voltage wiring. It should be shielded from
any capacitively coupled noise effects. If the cable runs close to any section of the apparatus that could be subjected
to a very rapid change of potential when a spark discharge occurs, then the probe cable may need additional shielding
to prevent capacitive coupling of the noise.
The retaining jack screws designed to hold the probe connector onto the teslameter must be screwed up finger tight,
as they form part of the electrical connection of the shield system. The woven braid of the probe cable is terminated to
the probe connector case. The retaining screws then connect the probe connector case to the teslameter case.
The teslameter itself should be sited in a sheltered location, where it will not be exposed to spark discharges or
radiated or capacitively coupled noise. The teslameter case is made of metal for shielding reasons. However, of
necessity it is less than perfect, as apertures have to be left in the case for the display and various connectors etc.
The unit is a precision measuring device, and should be treated with care, not subjected to adverse environmental
conditions.
The plugpacks supplied with each teslameter should be plugged in to a clean mains power supply. Noise on the mains
will work its way through the transformers and disturb the teslameter. Simple mains filters are readily available if there
is only one mains supply for the whole machine. Route the low voltage lead away from high current or high voltage
wiring. Ideally cut the low voltage lead to the minimum length required for the installation and re-connect the plug to it.
