Instrument transformer installation and maintenance- complete list of precaution and instructions.

Instrument transformer installation and maintenance requires high care with trained up personnel. As it’s an electrical equipment attached with sensitive equipment and system.
National legislation can set down the minimum age and the criteria for competence of skilled persons working on, with, or near an electrical installation.

1Always consider transformer as a part of the circuit to which it is connected, and do not touch the leads and terminals or other parts of the transformer unless they are known to be grounded.

2Always ground the metallic bases of instrument transformer.


3Always ground one secondary terminal of the transformer, except if the windings of voltage transformer are connected to open delta. Residual voltage windings connected to open delta must have dn terminal earthed only on one of three transformers (earthing screws at dn terminals of others two transformers have to be removed).

When the secondary of transformer is interconnected, there should be only one grounded point to prevent accidental paralleling with system grounding wire. In case of disconnection from the ground, the grounding screw has to be removed from the secondary terminal.

Connection between secondary terminal and base plate (ground) is shown on the picture “Cross section of double line terminal box“

4Always short-circuit the secondary of the current transformer, which is not currently in use to prevent secondary voltages which may be hazardous to personnel or damaging to the transformer’s secondary. The secondary like this must be additionally grounded.


5Never short-circuit the secondary terminal of a voltage transformer even this is not in use. A secondary short-circuit will cause the unit to overheat and fail in a very short period of time.

6Protection of single pole insulated voltage transformers against feroresonance phenomena is stated in appendix 3. – Damping of the feroresonance in Voltage transformers type TJC/TJP.

7In case of the current transformer with voltage indication (coupling electrode included) is secondary terminal box equiped with PE terminal, which is connected with earthing screw to the base plate, which must be generally earthed.
Attention: Terminal PE must be always earthed, this is hold generally, even if the base plate is removed. In case of disassembling the base plate, producer doesn’t warranting the earthing.

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