46kV Breaker Trip Coil, Trip Switch Contact Rating 2

[sialac]

We have purchased a Siemens SPS2 46kV breaker. We have a 125VDC trip circuit. Siemens lists the trip circuit draws 12A(Nom.) at 70-140VDC (125VDC Nom.). They Use a Square-D pushbutton (Class 9001 Type K-30 mm with type KA1 contact blocks) in their trip circuit to energize the trip solenoid in order to trip the breaker. A 52a contact from the breaker is wired in series between the trip pushbutton contacts and the trip coil and actually breaks the trip circuit (the pushbutton contacts probably would not break the circuit).

I asked Siemens for more information and they provided me with the following:

Contact Block:Class:9001,Type:KA1,Series:Q
Heavy Duty AC/DC
Thermal uninterrupted current 10A

From actual breaker test data:
Opening Time: 22-23 milliseconds
Max Coil Current: 10-10.5 Amps

Pushbutton is activated (circuit complete)
Trip coil is activated at approx 8 milliseconds
(circuit sees slight drop then continues up)
Current ramps up in approx 10-12 milliseconds to max amps
52a contact breaks current after another 10-12 milliseconds

Trip circuit does not see above 10.6 amps and circuit is
broken within 22 milliseconds of being initiated.
Max current through pushbutton of 10 amps occurs for
approximately 10 milliseconds before circuit is broken.

So, while Max current reaches to slightly exceeds rating
of the pushbutton, the whole operation takes place so
quickly that we do not see adverse effects on the pushbutton.

(end of Siemens reply)

They have insisted that the contacts on the pushbutton are good for 10A (AC or DC), but I have questioned Square-D and they tell me the contacts are rated NEMA Q600 for DC (2.5A continuous at 125VDC, 0.55A make or break at 125VDC)and NEMA A600 for AC (10A continuous at AC).

My info from talking with Square-D is that their 9001 KA1 contact blocks are NEMA Q600 rated (Series Q) for DC application.

Square-D admitted to me that there was an error in their on-line manual 9001CT0001.pdf page 149 that incorrectly lists the contacts as being 10A DC rated NEMA P600, and that they are in fact be NEMA Q600, 2.5A rated. The Q600 ratings are 2.5A (thermal continuous) and 0.55A make and break at 125VDC (69VA). Square-D does have the correct information published in their on-line catalog 9001CT0401.pdf page 7 showing the 0.55A, 125VDC rating.

I've looked through the NEMA ICS 5-2000 publication and don't see that the contacts would be rated to make or break 10.6A of coil current. Now I'm back to my question. How can Siemens use the Q600 contacts for DC application of 10.6A? I'm wondering if there is some sort of overload factor for the contacts that NEMA doesn't particularly take into account since we are dealing with such a small interval of time (23ms) that the contacts are in use during tripping.

Thanks

 

[dpc]

There are three different contact ratings - continuous, make and break. I'd have no concerns about the 10A continuous rating - the tripping current is very brief - or at least it should be. If it isn't you'll have bigger worries than the pushbutton.

I would be concerned if the pushbutton had to break 10 A at 125 v dc. But as you state, it doesn't have to do that provided the operator pushes it in long enough for the breaker to trip and allow the 52b contact to open up.

Most protective relay contacts are not rated to break the tripping current either.

To increase the breaking capability, you can put multiple contacts in series. To increase the making and continuous capability, you can put multiple contacts in parallel.

Or put in an interposing relay, or a control switch.

 

[stevenal]

Since the current through an inductor cannot change instantly, your make current is zero. Continuous duty is also zero since the device is rarely closed. And breaking duty is performed elsewhere making the contact breaking requirement - zero.

 

[sialac]

Thank you both. You have confirmed my suspicion.

To discuss a little bit more, does anyone know if there is a NEMA (or other) specification that indicates the short term (like, say, 1 minute) rating for these type of contacts? All I see in NEMA is make/break and continuous. I noticed the "traditional trip close switches made by GE and Electroswitch Series 24 have this type of rating listed.

Also, does anyone have any guidelines as far as the inductive number of contacts needed to parallel and / or series (like dpc mentions) for different circuit requirements, if all I have to go on is a NEMA rating to start with?

Finally, if the contacts are A600 for AC (10A continuous) and Q600 for DC (2.5A continuous), why is it that the continuous amperage rating is different for AC than DC?