Contact Rating?

[Kiribanda]

Gentlemen,

I have a question with reference to contact ratings. I have noted the following specification for output contacts of a relay.

“Breaking capacity (20,000 operations): At 250 V d.c. the contact rating is 0.25 Amps, L/R = 30 ms”

What does it really mean? How do we estimate L/R of a typical MV circuit breaker trip coil circuit?

Thanks in advance!

Kiribanda

 

[davidbeach]

That contact will not break the trip coil circuit of a MV circuit breaker. The relay manufacturers are expecting that another contact, usually a 52a contact, will break the trip coil circuit when the breaker opens. Check the relay instruction manual, I'm sure it will caution against allowing the relay contact to interrupt the coil (close coil also) circuit and the manual should suggest means of ensuring that the relay contact is not called upon to do so.

 

[buzzp]

I always like them breaker specs. I have not been in power long but have yet to see an MV breaker that tells you the make and break ratings of the contact that is supposed to energize or de-energize the trip or close coil of a breaker. In the UL world, this is called 'pilot duty' and specifies the VA switching capability of a contact, or if you will the make and break rating. It appears that your breaker does not give this either. I would interpret the data to be the continuous current rating of the contact and not the 'pilot duty' rating, if it has one.

I am not sure about estimating the L/R time but it can be tested using Doble or other types of test equipment. I suppose you could model the control system reactive components and resistances and calculate, electrically, the time but the mechanical parts will be the largest factor in effecting times.

 

[dpc]

I agree that the L/R = 30ms is a fairly useless piece of information. Trust that anything you try to interrupt related to a breaker control circuit is going to be highly inductive.

GE does provide tripping current for their vacuum breaker trip coil. At 125 Vdc, it's on the order of 10 A. At 48 Vdc it's about 25 A! There's a reason for those 30 A fuses.

I'd check with the relay contact manufacturer and get a better specification. This sounds like a Schweitzer output contact spec?

 

[Kiribanda]

Gentlemen,

Thanks a lot for all your valuable and prompt inputs.

I totally agree with everybody that this relay contact should not break the trip coil current.

I think I made a mistake by asking the second question at the same time with the first question. I should have asked the seconed one later.

Let us forget about the second question please.

So what does it mean by “Breaking capacity (20,000 operations): At 250 V d.c. the contact rating is 0.25 Amps, L/R = 30 ms”

As dpc pointed out is this a useless piece of information?

Normally L/R gives the dc time constant of an inductive load.

Kiribanda

 

[dpc]

Breaking capacity refers to the ability to interrupt the current. And yes, the L/R does refer to the time constant of an L-R circuit.

dc is much more difficult to interrupt than ac because the current never goes through zero like a sine wave does. Typically, the dc interrupting ratings may only be 10% of the equivalent ac rating. If necessary, you can increase the interrupting rating by putting multiple contacts in series.

The "make" rating of the contact - maximum current it can handle when closing will be much higher than the "break" rating. So generally, a relay contact is adequate to intiate a breaker trip or close, as long as there is another contact provided in the breaker to interrupt the current flowing in the trip or close coil.

 

[xabproject]

Dear Kiribanda,

Suggested reading - Article 2.6.3(c) from Van Warrington's Volume 2: Protective Relays - Their Theory and Practice.

For a project in the erstwhile SCECO(E) - Saudi Consolidated Electric Company in the Eastern Province, I was asked to do a complete application check on the usage of a number of specific contacts of certain important relays that were employed in important switching operations vis-a-vis the L/R considerations. How I walked that 'tight rope' and closed out the issue is another question

best regards,

 

[Bung]

It seems to be a description that has got into the standards (eg IEC 60255, but I think that one talks about L/R of 40ms), and nobody can quite remember why!

The contact can't be expected to interrupt trip or closing coil current, but it should be able to deal with any multi-trips it is connected to. If you are trying to drive a couple of old WLs with modern digital relay outputs, look out! Presumably multi-trip operating coils are designed with this in mind, but I've never seen anything that says it is so.


Bung
Life is non-linear...

 

[davidbeach]

Bung,

If by multi-trips you are referring to what we on this side of the puddle would call a lock-out relay (IEEE device #86, Electro Switch is a common manufacturer), they include clearing contacts in their trip circuits.

 

[ScottyUK]

david,

A trip / lockout relay is an 86 code over here too. Never heard of a multi-trip before. Hopefully Bung will educate both of us.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[dpc]

In talking with a couple of major manufacturers of digital relays, they both indicated that the most common problem they have with their protective relays are failed output relay contacts.

Contact arc suppressors are a good idea.

 

[davidbeach]

dpc,

If trip and close circuits are provided with proper clearing contacts, and the relay logic is programmed in such a manner as to prevent the relay from opening its output contact while that circuit is still energized, the contact arc suppressors will never have anything to do. They certainly won't hurt anything, but they won't do anything useful either.

On the other hand, if the circuit and the relay programming do not work as intended, an arc suppressor could prevent the failure of a relay output contact.

 

[buzzp]

I believe this is an area where no one seems to want to pay attention too.
We recently replaced a plant with SEL relays. I asked them about their VA rating of their contacts. They really did not have an answer other than to say the contact was designed to energize a trip coil on a breaker and make 30A maximum. They can only break 0.3A @ 125VDC. Their concern was minimal. Of course, the breakers we have are ancient and they only indicate the continuous current requirements, nothing about making and breaking.
I opted to not energize the trip coil directly with the SEL but used an aux relay, electroswitch LOR. This is normal practice anyway. Our set-up also uses the SEL to energize some close coils. I decided to use an old proven aux relay for this, the MG-6, even though it has the same ratings as the SEL contacts.
Most of my experience is in product design and my opinion of power engineers is they lack understanding of contact ratings, not only in protective relays but also control systems. This seems like a minimal concern relative to the overall project and hence is often overlooked and causes premature failure of many contacts.

 

[Kiribanda]

Gentlemen,

Thanks a lot for all your valuable inputs.

We had a valuable discussion!

Cheers!

Kiribanda :->

 

[davidbeach]

buzzp,

I have no idea how old your breakers are/were or how they might have been wired, but I would expect that the trip circuit of the breaker included a breaker 52a contact and that the close circuit included a breaker 52b contact. If so, and the relay holds its contact closed until the breaker contact clears the circuit, the relay contact will then be interrupting 0.0A when it opens. The additional relays, while protecting the protective relay, also add additional connections and possible failure points.

 

[Bung]

Multi-trip = 86. Except they don't necessarily lock out. Just a contact multiplier for the trip contact of the protection relay (hence multi-trip). I thought it was an almost universal term in the old Empire (Oz, India, SarfEffrika etc).

The problem with 52a contacts is that if they don't open before the protection relay drops out, you end up with the itsy-bitsy little output contact on the protection relay having to deal with the trip coil. Easily detected with old e-m relays - they either handled it okay because they had nice big boofy contacts, or the damage was esily visible. We have had a number of instances where the protection relay contact coils have been vapourised, and nobody was any the wiser until the next trip operation not got to the trip coil.

We now fit contact arc suppressors on any protection relay contact being asked to operate a CB trip coil directly. It's expensive, but the cost of the damage of the trip failures due to blown-off contacts has paid for quite a number of the devices. And they work - we tried them out on a paralleled pair of WLs (old 86's that have an impressive slug and need around 4A dc or more to operate) and couldn't raise even the tiniest blue flash on the interrupting contact of a small auxiliary relay.


Bung
Life is non-linear...