Fusing Situation

[tulum]

Hello again,

I have some 5kv switchgear (2 5kv fused S&C'c) that is to be used on a 2400v system. The spec says that we shall install medium voltage, general purpose, current-limiting fuses (5kv, Amp-trap, E-rated, CS-3 series fuses). The switchgear also has 3kv distribution class lightining arrestors on the line side of the switches.

My concern is that there are extra considerations that should be adhered to when specifying current limiting fuses of a higher voltage level on a lower voltage level (5kv on 2.4kv).

During a fault the current limiting fuses will produce a large resistance and hence a large overvoltage ESPECIALLY on a lower voltage system... (I believe this is because the fuse resistance is the same however, the fault current is usually more and hence the voltage transient is greater when a CL fuse is applied on a lower voltage than rated, please correct me if I am wrong

My question is (longwinded but..) how do I compare my lightning arrestor properties with that of the CL fuse properties under fault? What considerations are necessary when applying a CL fuse in this situation?

Regards,
Tulum

 

[dpc]

Check with fuse supplier. They don't like to talk about it, but they can provide a recommended voltage range for each voltage class of CLF.

You're right that overvoltage is a concern with CLF applied at voltages below their rating. My guess is that 5kV fuses will be fine at 2400V. Most 2400V systems are insulated at 5kV levels anyway.

 

[busbar]

Discussion with a Ferraz-Shawmut applications engineer may be time well spent.

 

[tulum]

Busbar,

I have now (after your advice) discussed it with a ferraz-Shawmut applications engineer. He says that the fuse is fine for a 2400v system...as long as all of the associated equipment can handle a temporary overvoltage on the order of 18000v - I.e. The max. permissible peak overvoltage for CL fuses as per ansi c37.46, Table 5.


With further digging, IEEE C62.22-1997 gives the following:

"In the event that arrestor damage does occur (due to CL overvoltages), an arrestor with a higher MCOV rating than would normally be applied could be required i.e.,conduction would start at higher arc voltages, reducing the number of arrester operations and, therfore, reducing the duty on the arrestor. Should conduction occur, the energy (joules/KV of rating) dissipated would be reduced."

I am using a Cooper Power Arrestor p/n UHG03040A1A1A1 which I believe (waiting for an updated spec sheet) has a MCOV of around 2.55kv rms and a TOV of 1.5xMCOV?

IEEE can't be sugggesting the rating should be increased to 18kv.... so what rating should I increase to...

I am going to contact Cooper Power systems to find out... in the mean time any suggestions are appreciated.

 

[tulum]

I have contacted Cooper and there are no problems with the application...

18000 less than front wave crest...

regards

 

[busbar]

tulum, I had given no thought to the surge aspect. The quoted C62.22 text seems akin to, “Use of ashtray is recommended with burning cigarettes.” Hopefully the Cooper folks can shed some light on the 18kV aspect [although, that is not entirely out of line with the usual 30kVBIL rating for 2.4kV systems.]