Ieee std 1159-1995, ieee recommended Practice for Monitoring Electric Power Quality

7.3.2 Ground terminals
There are two purposes for connecting to ground terminalsÑsafety and performance. The instrument should
be referenced at the same ground potential as the circuits being monitored. Caution should be exercised
since the instrumentÕs power supply equipment grounding conductor may be at a different potential than the
sense lead connected to the monitored circuit ground. If the power supply equipment grounding conductor is
internally connected to the instrument chassis and also connected to the safety/reference terminals (as is usu-
ally the case), ground loops and noise can result. The ground sense lead should be tapped to the ground bus
being monitored prior to connection. If a spark results, a ground loop exists.
Monitoring location
Voltage transducer 
current
Transducer
Substation Metering 
PTs 
Voltage dividers 
Bushing taps
Metering CTs 
Relaying CTs 
Overhead feeder
Metering PTs
Metering CTs 
Underground feeder
Metering PTs 
Voltage dividers
Metering CTs 
Service entrance
Direct connection
Metering CTs
Clamp-on CTs 
In facility
Direct connection
Clamp-on CTs 

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IEEE
Std 1159-1995
IEEE RECOMMENDED PRACTICE FOR
42
7.3.3 Instrument power and invasive monitoring
Instrument power is generally supplied by a single-phase three-wire outlet and a standard power cord. If the
circuit being monitored is the same as that supplying the instrument, the user should be aware of the effect of
the instrument on the metered circuit. Voltage change due to the instrument current draw is generally not
large but can be noticeable, especially on a neutral-to-ground measurement. If the instrument power supply
is protected by parallel, clamping transient voltage suppressors such as metal oxide varistors and avalanche
diodes, the instrumentÕs ability to accurately capture the disturbance will be compromised. The instrument
power should be supplied by another circuit, or, as allowed by some equipment, a dc source (battery). If
another circuit is used, ground loops should not be introduced and excessively long power cords or sense
leads should not be used. If a battery is used (and there is no grounding provided through the power cord),
the instrument should be properly grounded.