Slip-Ring Short Circuit Withstand Rating

[ettek]

Hello All, I have a question. I am commissioning 16 offshore cranes that have slip-ring assembles rather than cable twist. These slip rings are housed in a steel enclosure and provide 480V power to the crane and are rated up to 1,000Amps on the largest segments. The OEM has provided no documentation, nor approval by any certifying authority.
My question is two part.
1) Does anyone know of any rule or standard that requires slip-rings used for power distribution to have calculations and have been tested by some certifying body such as UL in regards to Short Circuit Withstand Rating?
2) I'm looking at these slip rings as simply a rotating bussbar, is that too simplistic. They are definitely part of the 480VAC power distribution system.

I'm not concerned where any reference to a rule or guideline comes from whether it be NEC, IEC, IEEE etc... at this point as I have to start somewhere and because OEM has failed to respond to my request for information this has become a rather urgent matter.

Thanks in advance for any assistance.
ettek

 

[mike_kilroy]

Title question does not match body of post?

Title question does not match body of post?

title questions short circuit value but post did not ask for that? I have worked with high power (upto 400a 480v) slip rings in past and never was asked about short circuit rating, which doesn't help you, but you asked:

1) none I was aware of as sales rep for these for a few years, other than some customers required UL or CE for their machines
2) there are speed limit ratings also but thinking rotating busbar sounds ok.

My suggestion for some answers though is this: ask some OTHER similar slipring mfgrs! You have concerns that MIGHT cause you to have to replace these expensive units with another brand, so I suspect you will get good reception asking the likes of these highly engineer oriented companies:

www.uea-inc.com/
www.moog.com/.../slip-rings/...slip-rings/compact-slip-ring-capsules/

 

[petersonra]

given the thing came from overseas it seems likely that whatever SCCR it has would be meaningless here.

I am inclined to agree that treating it like bus bar is in order.

Having said that, did the control panel come with a SCCR? If not, how do you get past that issue first?

 

[ettek]

Slip-Ring Short Circuit Withstand Rating

Thank you both Mike and Bob for your response. I will contact other slip-ring manufacturers and perhaps can generate some leads. As for Mike's question all MCC and Interupt devices are ABB so no problem there, cables are all UL approved and manufactured at an ABS certified facility. My concern is the weak link. Below is the letter which I wrote to the builder, if you have any suggestions I'm very happy to listen. Thank you for your time.

Dear Sir,
I believe the query is very specific and quite clear. We want to know the withstand rating of the slip ring assembly and it?s enclosure.

The withstand rating being the level of fault current which a non-interrupt (i.e. power distribution equipment such as buss bars, MCC?s and yes even slip rings used for transmission of power) device can safely handle, for a defined time, without failing. By failing I mean having any mechanical damage whatsoever no matter how slight it may be is still considered failure.
There is no arguing that the slip-ring assembly is part of the 480VAC power distribution system as it is in the current carrying path of the crane?s prime movers, the crane cannot operate without them.

We suspect that the slip-ring has very little withstand capability. If this is in fact true, it does not necessarily negate the use of the slip ring assembly. In this case once we have the withstand rating we will know if we are also going to be relying on another element of the system to protect it. In other words will the circuit breaker open before a damaging level of current at the slip ring is reached?

This is very basic but very significant question and should be easy to answer.

BRgds

 

[jim dungar]

FWIW,

The NEC (110.10?) requires that all equipment be applied within its SCCR.

 

[petersonra]

Thank you both Mike and Bob for your response. I will contact other slip-ring manufacturers and perhaps can generate some leads. As for Mike's question all MCC and Interupt devices are ABB so no problem there, cables are all UL approved and manufactured at an ABS certified facility. My concern is the weak link. Below is the letter which I wrote to the builder, if you have any suggestions I'm very happy to listen. Thank you for your time.

Dear Sir,
I believe the query is very specific and quite clear. We want to know the withstand rating of the slip ring assembly and it?s enclosure.

The withstand rating being the level of fault current which a non-interrupt (i.e. power distribution equipment such as buss bars, MCC?s and yes even slip rings used for transmission of power) device can safely handle, for a defined time, without failing. By failing I mean having any mechanical damage whatsoever no matter how slight it may be is still considered failure.
There is no arguing that the slip-ring assembly is part of the 480VAC power distribution system as it is in the current carrying path of the crane?s prime movers, the crane cannot operate without them.

We suspect that the slip-ring has very little withstand capability. If this is in fact true, it does not necessarily negate the use of the slip ring assembly. In this case once we have the withstand rating we will know if we are also going to be relying on another element of the system to protect it. In other words will the circuit breaker open before a damaging level of current at the slip ring is reached?

This is very basic but very significant question and should be easy to answer.

BRgds

I would bet its SCCR is a lot higher than your think, but is probably not UL listed or tested to US standards so whatever it is probably won't mean much when you try to apply it here.

 

[fmtjfw]

I would expect the weak link to be the "brushes" or whatever device transfers the power from the slip ring(s) to the movable part (crane). If they are carbon brushes you may be able to get some information from brush manufacturers or motor design texts.