Short term source paralleling exceeding fault rating of bus

[ScottyUK]

I’m developing an LV switchboard replacement programme and am in the spec review stage prior to going out to tender. I’ve identified what I believe is a problem with the fault rating of the original switchgear and would appreciate some thoughts.

The boards are designed as a three-section type with one incomer per section and a bus section switch between the A and B sections, and another between the B and C sections. There is no interlocking between the incomers and bus section switches giving rise to a condition where we have the potential for two or conceivably three sources feeding one bar with one or both bus section switch(es) closed, a situation which will take the fault level massively over the ACB rated breaking capacity and also exceed the rated capacity of the bus bracing. The normal operating condition is for the bus section switches to be open and in this situation the board is adequately rated.

Paralleling is necessary in order to avoid interrupting the process and is controlled procedurally. The switching process is a manual local operation. Paralleling only occurs for a short period when one source has to be taken out of service or is being returned to service. My opinion is that the bar and breakers must be rated for the maximum foreseeable fault. I think the condition where three sources are in service and both bus section switches are closed could be prevented by a 4-out-of-5 key exchange interlock, leaving the worst case as two sources with a closed bus section switch between them and the third bus section on a single source.

‘Others’ here believe that exceeding the rated fault level for the short term parallel operation is an acceptable risk because ‘that is how it has always been done’. I disagree and think there is no acceptable excuse for exceeding the equipment rating, particularly when modern gear is capable of meeting the requirement even if the old gear we are replacing, which is roughly 40 years old, couldn’t do so. I’m especially unhappy because it’s likely that I will be expected to sign off this design, and I’m going to raise a few eyebrows if I refuse to do so. I had hoped not to rock the boat too much in my new job until I’d had time to settle in but it seems I’m back to my old tricks.


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

 

[burnt2x]

"I can't totally eliminate the risk, but I can certainly minimise it."

I can't disagree with that!
I have read the landmark decision on a case in the UK regarding the Edwards vs. National Coal Board which states:
“Risk must be insignificant in relation to the sacrifice or time, money, and effort required to avert it.”
Having said that, I guess you're on the right track.

 

[racobb]

Scotty,

I don't know how anyone could justify intentionally exceeding the interrupting rating of the equipment, for any reason.

Alan

 

[Pikoplatt]

Would current limiting fuses installed in series with each source help reduce the through-fault current contribution to downstream faults on the buses connected via the bus-ties? The fuses can be selected for operation within their current limiting range, for faults on the buses connected by the tie-breakers. Although the fuse protection may not coordinate with downstream protection, they can possibly limit fault currents to,levels within the withstand rating and interrupting rating of the switchgear. These fuses would also clear the faults before the breakers attempt to do so, as fuse clearing time is less than relay/breaker operating time. This method poses the risk of fuse operation for faults on feeders connected to the main bus, so the consequences of a misccoordinated trip vs a total destruction of the main switchgear must be evaluated.

 

[Zogzog]

Scotty, FYI, we just opened an office in the UK for our remote racking and switching, they are designing remote operators for european switchgear.

 

[ScottyUK]

Scott -

Thanks. They got a phone number or website?

Pikoplatt,

Yeah, it's an option but I can buy an ACB with the required withstand, just some folks want "like-for-like" because "that's how it has always been". Well as far as I'm concerned it's always been wrong, so f*ck that for a solution. Not with my name on the design.



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

 

[Zogzog]

Scotty, I dont think I am allowed to post any contact info but I think I can post a link. The UK office is at the bottom of this page

http://www.cbsarcsafe.com/contact-us.htm

 

[ScottyUK]

Duly noted - I'll probably get in touch once we know what gear is going in and see if they can develop a remote racking solution for us.

Thanks.


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