Induced voltage from current transformer

[Yooper]

I was looking for some information regarding a question from an electrician at my facility.

We currently ground out all busswork while doing PM work on it. The reason being is obviously to avoid any induced voltages which could be harmful to the workers.

The question is: Is it possible to get enough of a induced voltage on relay control wiring to induce a voltage on the buswork through the CT?

Thanks

 

[RajT]

The simple answer is NO and therefore you need not disturb any wiring to the CT

 

[Yooper]

What is the not so simple answer?

 

[electricpete]

In most simple relay circuits the ct itself energizes the circuit. With bus deenergized the relay circuit (ac/coil side) is deenergized.

There may be more complex circuits (differentials for example) where multiple ct's feed the same circuit. Even in that case I would think the voltage possible on primary would be low for two reasons:
1 - there would usually be a low impedance path in parallel to the ct with deenergized primary/bus. (we don't put ct secondaries in series).
2 - Voltage is stepped down from secondary to primary. Typically ct saturation condiciton would correspond to about 1 volt on primary (bus).

 

[electricpete]

In matters of safety you should take it upon yourself to do a thorough independent review regardless of what internet guys like me say.

 

[DanDel]

Yooper, you have not mentioned the line voltage of the system or whether the relay circuits are the only live voltage present anywhere near the buswork upon which you are performing PM.
There are many ways for buswork to become charged with annoying and/or dangerous voltages.
The reason for grounding the buses before any work is to avoid any surprises while you are working. For this reason, I would ask you to continue what you are doing; it is the safest way to work.

 

[busbar]

As discussed by electricpete, compare a PT to a CT associated with the same bus. The turns ratios are in opposite proportion, making a reverse-fed CT in no way the hazard of a PT in a similar situation.

 

[scottf]

I agree with the "not possible" answer. It is not possible to induce a voltage of any appreciable magnitude by appling a voltage to the CT's secondary.

As was stated above, the CT acts as a step-down transformer when looking from secondary to primary. The actual voltage that would be induced is determined by the ratio. For example, let's say you have a 200:5A (40:1)CT. If there is 10 V induced on the secondary side of the CT, the resulting voltage across the primary of the CT is about 0.25V.

I believe your concern could be based on thoughts about voltage transformers, where there can be dangerous voltage levels induced on the primary winding, with small voltages on the secondary side.

 

[Yooper]

Thanks for all the input. This confirms what I thought but I thought I'd drop a note and see if anybody had any other ideas.

 

[davidbeach]

scottf's 0.25V on the primary of the CT would be across the few inches of conductor in the primary of the CT, not between the primary conductor and ground. It is the PTs that you have to be concerned with, best to keep the bus bars grounded.

 

[jbartos]

Suggestion: The high secondary to primary turns ratio of CTs does not produce a hazardous voltage on the CT primary side.

 

[scottf]

davidbeach-

Assuming the voltage applied across the secondary is to the same ground, the voltage induced across the primary winding is from primary to ground, at least the potential to ground is nearly the same as across the winding.
Regardless...the voltage drop is of no consequence.

 

[jbartos]

Suggestion: The voltage danger would then depend on equipment grounding of CT and surrounding conductive surfaces in addition to system grounding.