Trip Contact Not Rated to Trip Coil

[ThePunisher]

Hi all,

I received a complaint from our client which indicates that the external trip relay used to energize the trip coil is having under-rated output contacts.

This relay is Phoenix Contact Part #PSR-SCP-24DC/ESP4/2X1/1X2, coil rating 24Vdc, output contacts rated at 110Vdc, 6A continuous/inrush rating, 42W (T=40ms)interrupting rating at inductive load, suitable for SIL 3 application.

The circuit breaker is Powell Vac-AR, 5kV, 3000A with trip and closing coils drawn current is 3.2A.

The circuit is generally wired so that this trip relay output NO contact is in series with the trip coil (in parallel with the protective/lockout relay contacts) and the trip relay NC contacts is wired in series with the circuit breaker closing coil as a closing interlock. This trip relay is operated on a non-fail safe and contacts only operate if the relay coil is energized. The comment is based on 42W at 110Vdc comparison with 3.2A trip/closing coil rating.

I disagree with the comment technically because the relay output contact wired to "close-In" or "make" to energize the trip coil which is rated at 3.2A. Since the output contact is rated at 6A, it should be ok. Furthermore,the trip or closing coil is de-energized by 51a and 52b simultaneously after the breaker operates.

Consequently, this also applies to the NC closing interlock contact on the closing coil circuit.

Do you agree with me? any thought otherwise?

 

[pwrtran]

The problem is trip and close coil have relatively large inductance that cause transient current or inrush current. The steady state rating of the relay contact may net be able to break the current. Options might be:
1) use certain SEL relay that has hybrid outage contact designed for coil/close coil application
2) or, implement and drop-off delay so that 51a pellet or 51b pellet open fast than the relay contact to make the heat of the inrush.

 

[davidbeach]

As long as the Phoenix contact never attempts to interrupt the trip circuit it will be just fine. Most relay output contacts used to trip breakers are unable to interrupt the trip coil current, but they never need to. If that isn't sufficient, look at a suppressor such as the SEL-9501 or 9502; it takes care of the inductive current.

 

[dpc]

I basically agree with David on the current rating - most protective relay contacts are not rated to interrupt tripping or closing current. But what is the nominal dc voltage of the trip circuit? The relay contacts are rated at 110 V dc. If this is a 125 V dc system, the voltage could be as high as 140 volts during equalization. I doubt it would be an issue, but you might want to verify the maximum allowable operating voltage across the contacts. BTW, 3.2 A tripping current actually seems pretty low.

 

[ThePunisher]

dpc, the nominal dc voltage of the circuit is 125Vdc. The maximum switching voltage on the relay is 250VAC/DC...it should be fine.

David, dpc, do I really need to be concerned about this interposing relay interrupting the circuit breaker trip coil current if the breaker trip coil is wired in series to 52a contacts? The 52a contacts will interrupt the trip coil current flow after the breaker opening is completed. The input signal to this relay (Process Shutdown) is latched or maintained type..hence the contact remains closed even when the breaker opens.

The only concern I can think of are the following:

a. If the breaker fails to open and the trip current will be continuous, and then the relay resets.

b. The relay resets or drops out (due to low DC voltage or loss of DC), before the breaker opening is completed.

For 'a', we may be able to make some changes to the DCS logic such that a 52a contact will be added such that DCS will not be able to reset after a process shutdown signal is delivered if the 52a contact is closed. In addition, arc suppressors like the one suggested by david will be installed on the relay trip output.

For 'b', this is an inevitable scenario...probably, wiring this relay to the circuit breaker lockout should work. However, it might be a nuisance to operations to come over and do local reset for a breaker trip caused by the process SIS.

How about using the numerical relay input/output contacts and configure the output contact to directly trip the circuit breaker and activate the "seal-in" feature of the relay output contact (similar to GE F60)? Any thoughts?

 

[rasevskii]

@thepunisher:

Perhaps you ought to listen to your client. As was said above, it is more likely that the trip coil of the CB draws more like 10A on a 125V system. Do not play games with tripping circuits. Use a man-sized interposing relay after the Phoenix (such as the old ABB type RXMH or RXMVB latching relay). A melt down of the phoenix inside the electronic rack will destroy a lot of surrounding expensive goodies and may cause a fire. Saw it happen at least once.

Just some advice from the field.

rasevskii

 

[pwrtran]

You don't need an interposing relay if the trip coil in series with 52a pellet giving 52a is rated for interrupting the current.

You also need CBF protection if the breaker fails to open.

 

[davidbeach]

No need for an interposing relay. Thousands and thousands of "wimpy" contacts trip circuit breakers all the time. It's all so fast, and cleared by the breaker 52a contact, that the only ratings of the tripping contact that really matter are the make rating and the voltage rating. Make current is a mechanical rating, carry current is a thermal rating, and interrupting current is based on how far apart the contacts get how fast. The suppressor takes care of the interrupting if things go horribly wrong and the trip contact tries to break the trip circuit. Carry isn't generally much of a concern since the trip coil will probably burn out faster than the output contact will fry if the circuit isn't interrupted by the breaker opening.

 

[cranky108]

Have you looked at the length of time the contact remains closed? If it attempts to open prior to the 52A opening in will have problems. Another thing sort of discussed above is some sort of conditioning arount the contact, such as a surge supressor, capacitor, or arc snubber.

There is a concern if the breaker mechinacally hangs up that the contacts that closed to trip the breaker will be damaged attempting to open the trip coil current. I don't believe there is a solution except to do better breaker maintenance, bigger contacts, or an electronic interrupter with the existing contacts. It's a bad condition to have to remove protectine devices because the contacts were damaged by a stuck breaker.

It is a simular case with a slow breaker, and no one will admit it is slow. It will test good, after it is loosened. And will open and look like it work as expected. Only good relay event files will show the truth. But once the fault current is cleared by other devices, the relay contacts will release and then may be damaged.

 

[stevenal]

Is this part of the protective relay circuit? If so, I would use something that meets all the relevant C37.90* standards.

Even relays built to these standards are not expected to interrupt trip coil current. A breaker auxiliary switch that has worked loose is another situation that will fry them. Crew:"The relay burnt up, we replaced it." Me:"Please return and inspect the auxiliary switch."

 

[dpc]

It makes sense to use a high-quality relay for breaker tripping. These are energize-to-trip systems and if the relay decides not to operate, things can get ugly really fast.

I really try to avoid any unnecessary interposing relays in trip circuits, but sometimes they are unavoidable. I also never use a plug-in relay in a trip circuit.

As long as the breaker auxiliary contact is working properly, the breaking current rating of the relay contact should not be a factor. But when I've talked about relay failures with manufacturers of new digital relays, the most common failure involves the trip circuit output contacts. And these are products are intended to be used to trip breakers.

 

[stevenal]

dpc,

I would bet that nearly all of those trip output "failures" was really a 52a failure that took out the output contact.

I agree the use of interposing relays should evaluated carefully. Added complexity, failure points, tripping delay, ...

 

[davidbeach]

Throw in PRC-005 considerations and those interposing relays go from being a bad idea to being a horrible idea.

 

[cranky108]

What I have seen is the relay guys do not work on breakers, and the substation guys do not work on relays, so a bad relay contact gets replaced, but not the breaker 52A.

 

[burntcoil]

Normally open contact in series with trip circuit looks fine but the NC contact in series with Close coil looks to me a strange choice. If you are going to introduce some interlock in closing circuit, this should be for some specific conditions which should be met before the circuit breaker can be closed. These conditions are mainly governed by human and equipment safety. If you want to use an interposing relay as a Close permissive, use heavy duty machine tool relays.

To me contacts from same relay being used both in trip and close circuit is also a concern.

Also in your question I do no see if you had mentioned about OPEN/CLOSE switch, Local/Remote selector Switch? Other important thing is that from where you are energizing this interposing relay, are you using it for remote trip from SCADA or a push button installed on switchgear door?

 

[davidbeach]

Normally open contact in the trip circuit and normally closed contacts of the same device in the close circuit is very common and quite necessary. The normally open contact trips the breaker and at the same time the normally closed contact opens to break the close circuit. Look at the typical wiring of a lockout switch (86); every breaker gets a set of trip contacts and a set of close block contacts. While the breaker is closed, the close circuit is interrupted by the breaker 52b contact; the normally closed contact of the device in question will be open before the 52b closes, so the close circuit never completes. Otherwise there is a risk of opening the breaker while there is a standing close and have the breaker pump. Yes, there should be anti-pump circuitry in the breaker, but it is so simple to block pumping in these conditions that it should always be done. Also, there is no telling where a different close signal might come from so just block them all.

 

[dpc]

Regarding blocking of the close circuit - I agree with David - this is the normal practice in the USA that I'm familiar with. Could be a bit of "belt and suspenders" in some cases, but it seems to have worked well over the years. This is especially true when there are multiple close contacts such are remote control, reclosing relays, SCADA, hackers and who knows what else.