Breaker trip/close interposing relays

[trat1208]

Hi all,

I am designing a project and am coming across some frustration dealing with interposing relay contact ratings. Any help would be appreciated.

Problem 1. I have a 4kv switchgear breaker I am replacing the relaying on. Due to some design considerations, I need to use interposing relays in the trip & close circuits for user-initiated / manual breaker operations (protective relays will be tripping breaker directly). My nameplate trip coil amps are 15.5A @ 48Vdc and nameplate close circuit amps are 3.6A @ 240Vac. Finding a plug-in relay with a 3.6A @ 240Vac contact rating for the close is pretty easy (Potter-Brumfield KRPA or KUEP). I am having difficulty finding a relay rated for 15.5A @ 48Vdc. In the past, I probably would've just thrown in a KUEP because I've seen these used often in the past for breaker trip/close SCADA interposing relay applications, but I was surprised to see the 10A rating when I looked it up and am questioning whether it is appropriate. The need for a 15.5A or greater contract rating does seem a little hokey to me though, since the relay will make and carry this current for about all of about 0.1sec and the 52a contact will break the circuit when the breaker opens. The KUEP datasheet doesn't give any guidance as far as momentary ratings though. Does anyone have any experience with spec'ing interposing relays like this, or can suggest a particular relay that would satisfy my application?

Problem 2. Another somewhat related aux relay question. Manufacturers specify a min recommended contact load rating in their relay datasheets. I understand the basis for this rating, wetting / breaking the film off the contact to get a good electrical connection when the contact makes. But this is usually specified at a very low voltage, i.e. 300ma @ 5Vdc or 12Vac, etc. Should I be extrapolating this (based off calculated VA?) to determine what I would need at a higher voltage like 48Vdc? Or else is there a voltage level where the min contact load rating is no longer relevant? These ratings seem difficult to deal with for applications where you want to use a contact to pick up a relay digital input that only draws <10mA (SEL relay). Again, I haven't had issue with this in the past and would just use any old plug-in relay, but unfortunately I can't unsee the datasheets I looked at the other day and this is bothering me.

Thanks in advance for any help. Was also planning to call the manufacturer tomorrow, but was wondering what people might have to say from their experiences.

 

[davidbeach]

Your interposing relay only has to make the trip (or close) current, it doesn't have to "carry" it and you certainly don't ever want it to have to break the current. The break is provided by the much more robust aux contacts on the breaker. Carry is a thermal rating, and when the current in question is only there for a few cycles, there's no heating to speak of. Close generally is present longer than trip.

 

[JG2828]

The ABB MG6 relay is the most heavy-duty interposing relay I can think of.

https://library.e.abb.com/public/d991229a370fa944c12574ce00550e8d/41-753.1N.pdf

 

[trat1208]

Thank you for the responses. I am glad that you concur that this "momentary" current draw shouldn't really be used to rule out relays with lesser continuous amp ratings, that was pretty much my take as well. And I had been keeping the thought of using an old beefy relay like the MG-6 in the back of my head as an option of last resort, but would really prefer not to have to put a big honkin relay in there for an application where I've seen ice cube relays used in the past.

Can anyone shed some light on the second part of my post, though? This recommended min contact rating seems very problematic to me. I.e., a SEL relay digital input (and I'd imagine similar for non-SEL microprocessor relays) draws like 5-10mA @ 48Vdc. This makes it difficult to spec out a general purpose relay (by any manf that I've seen) that would be rated to be able to reliably pick up a relay digital input. This just seems odd to me as I feel like I've seen general purpose plug-in relays used in this way in the past. Am I overthinking this? Or does this min contact load just matter less at voltages above the minimum shown in their spec (i.e. 300mA @ 12Vdc min load for a KUEP, vs 5mA @ 48Vdc for my application). I know there are reed relays available for lower level signals, but honestly have never seen these installed in utility applications with a station battery.

Thanks again.

 

[davidbeach]

In practical terms, based on thousands of relay-years of experience, your problem #2 isn't a real problem. Maybe we haven't seen the right set of circumstances to make it show itself, but off all the problems out there this isn't one I've encountered. If you want that circuit to draw more current you can always parallel a resistor to the relay input and get the increased current through the resistor. What that very low current does permit, though, is wiring a relay input in parallel with trip output to monitor the trip circuit, bleeding a small current through the trip coil. Multiple inputs all in parallel into a single trip coil don't present a problem; at higher currents they would.

 

[dpc]

I agree with David, but would just add that the vast majority of these relay-years of experience are based on operation at 125 V dc. If you're trying to do this at 12 V dc or 24 V dc, the data may be different.

 

[davidbeach]

Those aren't substation voltages... ;-)

 

[ppedUK]

A word of caution when switching DC on relay contacts - use hermetically sealed relays, or relays with perfect dust seals, not just "any old plug-in relay"!

The positive pole of the contact attracts dust over time and seems to occur at DC voltages above around 40V. The higher the voltage, the faster the build-up.

I worked on the design of a plant some years ago which had 48VDC control and re-start buses in the MCC control circuits. It all worked fine until the plant was around 5 years old, and then we found that some drives would not start from the Control Room, even though the start relay was energised. This then happened on a regular basis, with the only cure being replacement of the start relays (which were the "Ice cube" type but not sealed). Turned out to be dirt on the contacts, and no amount of load (these were switching motor contactors) would burn through the build up. This didn't seem to happen at 24VDC and below. We changed to hermetically sealed relays and the problem disappeared.

Protection relays and tripping relays etc. usually have good seals.

 

[DM61850]

SEL relays often have a set of contacts that are high current interrupting and you often can get by without using an interposing relay. Utilities often don't want to use an interposing relay because it is a single point of failure. Some places don't even both with using high current interrupting contacts for tripping because they always have 52A breaker contacts in series with trip circuit. The breaker contacts should be breaking the trip circuit during normal operations.

You only interrupt the trip current if the breaker didn't open or your trip coil didn't burn out. You have a lot bigger problems if either of those happened than burning out a contact on a relay. When I see someone putting a timer on how long to hold the trip contacts to avoid burning out the trip coil if the breaker doesn't open immediately, I get worried that it will fail to operate as intended. A slow operation to me is way better than no operation. To me, a trip should hold until the coil burns up or it operates because under protective trip conditions, it is that important for the breaker to operate.

 

[dpc]

Those aren't substation voltages...

David - I'm probably getting senile, but I'm not sure who you are responding to or what point you are making?

Regarding SEL output contacts - the number #1 point of failure in SEL relays is the output contacts. I'm sure this is true of every other digital relay as well. Transient suppression is good idea.

 

[catserveng]

I have done field work for a number of switchgear manufacturers over the years, older gear used a lot of the older GE HGA style relays, very robust, but large and expensive, but also had the option of a fast operate time.

I number of companies I have dealt with last several years use the P&B KUEP 3D15 X form factor relay with the magnetic blowout, I have installed a large number of these in retrofit projects also with no problems I know of in systems using 48VDC and 125VDC nominal voltages mostly on vacuum breakers rated 5 and 15kV from Siemens, SquareD, GE (both PowerVac and PowellVac) and the Westinghouse (Eaton) VCP breakers.

Hope that helps, MikeL.

 

[davidbeach]

125 VDC is a good substation voltage. 48VDC is a relic of building on the cheap. 12 or 24VDC is a bit wimpy. Yeah, I know all sorts of things get done, but skimping on DC voltage and therefore skimping on battery costs may be false economy. But that's just me...

 

[capnron]

As noted in the previous posts the HGA or MG6 relays are time tested and long lasting in the circuit type you are talking about. Wire 2 sets of contacts in series if you are worried about breaking the DC tip current.