Ampmeters and CT Grounding

[tom baker]

For a typical 0-5 AAC ampmeter and CT (used to show motor current) does one side need to be grounded?
I recall installing CT Overloads that required one side to be grounded, not sure about panel ampmeters.

 

[SG-1]

Tom, generally it is best to ground one side of the CT. This keeps the secondary voltage as close to ground reference as possible. Without grounding the voltage will float willy-nilly above the ground reference.

The only type of CT that I know of that is not normally grounded is an auxiliary CT.

 

[tom baker]

That makes sense. I have about 50 panel ampmeters with CTs installed a few years ago. About 10% have failed. If one side is grounded, that will stablize the secondary...

Thanks

 

[dereckbc]

Tom not familiar with your application, but in Substation Relay Control, no CT's were ever referenced to ground as that would be very dangerous and impossible to work on, even those used in 345 KV distribution systems with 600:1 ratio. They are wired directly to the Transducer input on the SCADA.

 

[SG-1]

Is it possible to determine how the ammeter failed ?

Did the meter develope an open circuit or a dielectric failure ?

I am not familiar with high voltage applications that Dereckbc mentioned, but I am familiar with medium voltage switchgear applications. The CT are almost always formed into a wye with the neutral taken to the switchgear ground bus. This is done for safety. These bushing type ( donut ) CTs are impossible to work on unless the gear is de-enegized.

When we mount POCO inline CTs some of the POCOs require a 6 gauge to the ground bus.

 

[SG-1]

Tom, another thought occured to me that if the ammeters are seeing a lot of inrush, like on a motor they my not be rated for the overload that they experience.

Are these electronic or electro-mechanical type ?

 

[Thedroid]

All of the CT's I've seen had x2 grounded.

 

[SG-1]

Either X1 or X2 can be grounded. It depends on the physical polarity of the transformer & the desired electrical polarity to the device. They can be physically installed such that X2 is always grounded, but they do not have to be.

 

[don_resqcapt19]

I have never seen a grounded CT circuit that was used for current display on a panel meter. The ones I have seen take the two wires directly from the CT to the meter terminals.

 

[mivey]

Even CT circuits are grounded. The key is to ground only in one place to avoid a ground loop.

See IEEE Std C57.13.3 "Guide for the grounding of instrument transformer secondary circuits and cases".

 

[mivey]

IEEE Std C57.13.3-2005
7. Exceptions to grounding
Exceptions to grounding are permissible and sometimes required where advantages obtained by not grounding (in certain instances or in certain types of installations) are considered to outweigh the safety or other advantages obtained by grounding. Such exceptions should comply with the recommendations of the NEC. Grounding exceptions are generally permissible under the following circumstances:

a) If the primary windings of instrument transformer circuits are connected to circuits of less than 1000 V with no live parts or wiring exposed or accessible to other than qualified persons, the circuits may not be grounded.

b) For instrument transformer cases, the cases or frames of CTs may not be grounded if the primary windings are not over 150 V to ground and are used exclusively to supply current to meters.

c) Cases of instruments, meters, and relays operating at less than 1000 V on switchboards having exposed live parts on the front of panels are not usually grounded. Mats of insulating rubber or other suitable floor insulation are provided where the voltage to ground exceeds 150 V.

d) Instruments, meters, and relays, whose current-carrying parts operate at voltages to ground of 1000 V and over, are isolated by elevating them or protecting them by suitable barriers, grounded metal or insulating covers, or guards. In such situations, the cases are not usually grounded.

WARNING
When exceptions to grounding, as suggested here, are considered for implementation, the reader should make certain to meet requirements stipulated in the NEC and NESC. Article 90.2 (A) of the NEC lists the installations that are covered by the Code, and Article 90.2 (B) lists the installations that are not covered by the Code. For immediate reference of the readers, Article 90.2 of the NEC is reproduced in Annex C.

 

[mivey]

IEEE Std C57.13.3-2005
4. Need for grounding secondary circuits of instrument transformers and cases
It is necessary to ground secondary circuits of instrument transformers for protecting the protection, monitoring, and control equipment that are connected to them and for the safety of personnel who might come into contact with them. The primary circuits of instrument transformers are at high voltages; in many cases, they are in the range of hundreds of kilovolts.

There is a (stray) capacitance between the high-voltage circuit and the secondary circuit of the instrument transformer. If a secondary circuit is not grounded, there will be a capacitance between the secondary circuit and the ground. The capacitances between the high voltage circuit and the secondary circuit of the instrument transformer, and the capacitance between the secondary circuit and ground, are now connected in series and act like a voltage divider. A charge could accumulate on the secondary winding of the instrument transformer.
...
If the secondary circuit of an instrument transformer is grounded, the capacitance between the secondary circuit and ground is short-circuited and electrostatic potential does not build up on the secondary circuit.
...
Similarly, there is a (stray) capacitance between the primary circuit and the case of a device...

 

[mivey]

I have never seen a grounded CT circuit that was used for current display on a panel meter. The ones I have seen take the two wires directly from the CT to the meter terminals.

There may have been a shorting bar that made the connection to ground with shorting screws that you did not notice. See figure 4 here:
http://www.nktechnologies.com/pdfs/ct-tutorial.pdf

 

[don_resqcapt19]

There may have been a shorting bar that made the connection to ground with shorting screws that you did not notice. See figure 4 here:
http://www.nktechnologies.com/pdfs/ct-tutorial.pdf

Nope...most of these are two wire donuts with the wires from the donut extended directly to the terminals on the display meter.

 

[dereckbc]

There may have been a shorting bar that made the connection to ground with shorting screws that you did not notice. See figure 4 here:
http://www.nktechnologies.com/pdfs/ct-tutorial.pdf

I am only speaking in reference to utility sub stations but shorting bars are used to short out the windings to the CT, not to ground per se.

CT current ratios are on the order of 200:1 to 600:1. Of the 4 cases or people I know who have lifted a CT wire on a live circuit without using the shorting bar, only 1 survived if that is what you want to call it.

 

[mivey]

Nope...most of these are two wire donuts with the wires from the donut extended directly to the terminals on the display meter.

Did it meet one of the exceptions from my post #11? If not, it was not wired per the IEEE standard.

 

[mivey]

I am only speaking in reference to utility sub stations but shorting bars are used to short out the windings to the CT, not to ground per se.

The bars can be used for both functions. In the figure I referenced, the strip is configured for grounding. Using the spare screw adds the shorting function.

CT current ratios are on the order of 200:1 to 600:1. Of the 4 cases or people I know who have lifted a CT wire on a live circuit without using the shorting bar, only 1 survived if that is what you want to call it.

As for other "lifting", I have seen others with broken CT wires, bad shorting blocks, etc. Some CTs "survived", all personnel survived (not to minimize the danger as I'm sure there are cases where the personnel did not survive-a good reason to not assume the safety device worked as designed).

I have seen some poorly manufactured metering terminal blocks that would not automatically short the CTs. I have also seen some metering locations where the jumper at the CT was never removed and the meter had been incorrectly registering for years.

 

[dereckbc]

Did it meet one of the exceptions from my post #11? If not, it was not wired per the IEEE standard.

There is no requirement any where within a jurisdiction to meet IEEE standards. IEEE standards are completely voluntary.

 

[dereckbc]

As for other "lifting", I have seen others with broken CT wires, bad shorting blocks, etc. Some CTs "survived", all personnel survived (not to minimize the danger as I'm sure there are cases where the personnel did not survive-a good reason to not assume the safety device worked as designed).

I can only assume they were not working on 135 to 345 KV transmission lines. With a 600:1 current ratio on 345KV, an open CT voltage rises to 207,000 KV in a pico second when opened with 1 to 10 amps capability through the arc flash in theory. That is one large explosion.

 

[mivey]

There is no requirement any where within a jurisdiction to meet IEEE standards. IEEE standards are completely voluntary.

That is true, if the utility can properly justify whatever they practice. And they usually can.

Nevertheless, for most of the higher voltage applications we will see here, it is standard practice to ground the non-polarity secondary lead of a CT.