Switchgear Fault Level
Switchgear Fault Level
(OP)
What is the remedy for a switchgear when its fault capacity is exceeded? Let's consider a case where generators are added to a busbar without considering the fault capacity of the board.
Is there anything that can be done to rectify the situation?
Thanks for your thoughts.
Is there anything that can be done to rectify the situation?
Thanks for your thoughts.






RE: Switchgear Fault Level
http://www.gwelec.com/product_current/clip.cfm
RE: Switchgear Fault Level
Therefore, you must conduct load flow study as well as short circuit current study to verify whether there will be serious voltage dips on system.
RE: Switchgear Fault Level
http://www.tyndall.ac.uk/publications/working_papers/wp25.pdf
RE: Switchgear Fault Level
I understand that only the manufacturer is allow to replace the internal components.
RE: Switchgear Fault Level
Presume you can not mitigate the high fault level by, say, opening a bus section switch. Additional generation may also increase supply security and permit this. Finally it may be worth contacting the switchgear manufacturer to see if they can change some components to enhance the fault rating. I've experience of doing this a few times on older types of switchgear
Rod62
RE: Switchgear Fault Level
However, it does take a specialist to perform fault current study and coordination and it is a single phase device, although most motor starters should have already have single phase protection already.
RE: Switchgear Fault Level
How close can your fault capacity get to the fault rating of a switchboard. Let's imagin that a switchboard is rated for 30KA for 3 seconds, what fault capacity can you have on that switchboard to be safe?
RE: Switchgear Fault Level
RE: Switchgear Fault Level
This is simply the rated short time current of the of the board (busbars etc.). The amount of current that the board can handle depends on the duration of the fault i.e. k=I^2t = 2700kA^2s. If the max. fault duration was 2s then rated short time current(2s) = sqrt(2700/2) = 36,7kA for 2s.
What is normally more important than the above is the 'make' and 'break' ratings of the circuit breakers. The 'break' rating would typically be the same as the rated short time current (30kArms) and the make on an IEC breaker would typically be 2,5 x 'break' = 75kApeak.
Careful consideration needs to be given to the asymmetry during the fault and any contribution from large motors. It is no good using steady state fault currents if the protection will trip instantaneously.
In other words be VERY careful.
The most cost effective solution would probably be fault limiting reactors. As mentioned in another post volt drop needs to be considered, although this should not be a problem unless large motors are being started DOL (still, it needs to be considered). A calculation of the cost of the reactor losses should also be carried out before a decision is made.
Current limiters are very expensive and tend to compromise protection grading.
As an outside chance, are there no bus sections that can be run open ?
RE: Switchgear Fault Level
How about using an interposing transformer for the generator ?
_______________________________________
Regards -
Colin J Flatters
Consulting Engineer & Project Manager
RE: Switchgear Fault Level
Thanks for the valuable post.
Fault level is regarded the available current after normally 8 cycles. This is the value that is compared against the short circuit capacity of a switchgear. I know that the protection may break the fault current sooner than 3 seconds, however if the available current is close to the short circuit capacity of the switchboard then we must consider rectifying the problem.
Lets say we have come close to the limit for this switchgear. Adding more generation capacity to this can not be considered unless rectification measures such as fault limiters are used.