Medium voltage cable installed through CT's

[LAYMAN JOE]

35kv cables terminate at a fused Air Switch. These cables go through separate CT's for metering ONLY. This is not service equipment. These CT's are not for the protective relay system, they are only for metering. When cables go through CT's for protective relays you must always take the drain wire back through the CT before grounding it.

There is an ongoing debate as weather to take the drain wire back through the CT for metering. Many experts I know say you DONT take it back through for metering and many other experts say that you always take the drain wire back through for every application.

Oh how I hope someone on this board would enlighten me.

 

[LAYMAN JOE]

bump.........

 

[wirenut1980]

I believe the purpose of the drain wire is to bleed off charge buildup at a dead front termination (at least in my experiences). And I do not think that any current through this drain wire should be measured for billing. So IMO, the drain wire should be run back through the CT.

 

[vhay]

The reason that the drain wire is routed back through the CT for protection circuits is that current flow in the drain wire will affect the earth fault protection. In the case of metering CT's, the $'s are affected and drain current will not be captured and paid for. It is probably not a lot of money, but a penny saved.........
Vern

 

[LAYMAN JOE]

I think you guys are onto something here...

Route the drain wire back through the CT= Utility pays
Dont route the drain wire back through the CT= Customer pays.

That being said, The customer should pay for power used and therefore you should not route the drain wire back through the CT.

Yea or neh?

 

[jim dungar]

The drain wire is connected to ground. Any ground current flowing over the drain wire will affect the amount of current the CT measures (maybe more, maybe less). This is definitley not something you want to have happen if the CTs are being used for protection, especially differential protection. For example, imagine a ground fault current negating the phase current - the protective relay would not see the problem.

 

[LAYMAN JOE]

Indeed! You MUST ALWAYS take the drain wire back through the CT when the CT's are for any protective relays.

What would you do with the drain wire if the CT's are for metering ONLY?

The drain wire is connected to ground. Any ground current flowing over the drain wire will affect the amount of current the CT measures (maybe more, maybe less). This is definitley not something you want to have happen if the CTs are being used for protection, especially differential protection. For example, imagine a ground fault current negating the phase current - the protective relay would not see the problem.

 

[LarryFine]

Indeed! You MUST ALWAYS take the drain wire back through the CT when the CT's are for any protective relays.

What would you do with the drain wire if the CT's are for metering ONLY?

I would say the same thing. I can't think of a good reason not to.

 

[iwire]

I thought Jim D just made clear the drain wire can not run through the CTs ?

Looks like we read what he said in opposite ways.

Let me ask this.

Would you run an EGC back through the CT of GFP protection on a 480 service?

Of course not as it could make the GFP blind to a ground fault.

 

[jim dungar]

Indeed! You MUST ALWAYS take the drain wire back through the CT when the CT's are for any protective relays.

What would you do with the drain wire if the CT's are for metering ONLY?

This is not a code/standards issue it is a design concern.

If all you really wanted was to know was amps for an analog meter and did not care about accuracy then the drain wire may not be important. Digital metering (especially harmonics) may not be as forgiving.

After the installation (assuming the drain wire location is not documented) every one is going to "see" metering CTs and not realize how inaccurate they may be. Adding power monitoring equipment is difficult enough, in medium voltage equipment, without having to worry about installation.

It sounds like you are trying to justify an installation and not weigh the negatives.