SMU/NX to Bay-O-Net coordination 2

[fredpar]

For medium voltage [13,2KV] underground distribution which type of fuse coordinate better with the Bay-O-Net type? SMU-20 50Amps or NX-50?
Let's say I'm 2 to 3 miles away from the sub.
Thanks!

 

[davidbeach]

What does your coordination study show?

 

[fredpar]

Well my question is in general: What type of fuse do you use when coordinating with Bay-O-Net type fuse. I understand some people use smu type, others use nx type and so on. The same way for overhead some people like to coordinate with only type "T" and other with type "K", etc,. I just want to hear pro's and con's from the experts in this forum.
Thanks!

 

[DanDel]

You have to look at the curves.
By the way, a Bay-O-Net fuse can actually be composed of two different fuses (dual element), with a CLF backup in series in the 353C or 358C.

 

[jghrist]

In the high fault current range, I would expect the SMU fuse to coordinate better because the NX current limiting fuse will be current limiting at high fault levels. You may want to sacrifice coordination at high fault levels, though, to limit the fault current in the tranformer for a primary internal fault and prevent catastrophic failure.

 

[fredpar]

Dandel:
The name bayonet means dual and it comes from the rifle with a knife over the muzzle used in the mid 17th century for close combat.

 

[DanDel]

Thanks for that info, fredpar, but I wasn't sure you were familiar with the fact that they are a dual element fuse, and therefore even more difficult to coordinate, which is why I mentioned that you really have to look at the curves to determine proper coordination.

 

[apowerengr]

fredpar, I'd offer that an SMU or NX would be called for based on the load current you need to carry and the fault current you are trying to interrupt. SMU's are limited to somewhere in the range of 12-14kA, NX's up to 50kA. SMU's can go to 200 or 400 amps where as the NX is limited to roughly 100 amps.

Application questions could include load and fault current requirements. Coordinating with fault indicators could also drive you toward an SMU instead of an NX. If you plot the curves remember the bayonet is limited to 3.5kA or less (depending on voltage level) and is intended to clear secondary faults, not all primary faults. As mentioned backup current limiting fuses may be required or recognition that the upstream device may have to clear primary faults because the bayonet does not have enough interrupting rating to handle it. The previous statement assumes you are coordinating a bayonet applied in a transformer for transformer protection as opposed to the installation of some manufacturers inexpensive padmounted fused sectionalizing switchgear using bayonet fuses. Personally, I don't like the cheap padmounted switchgear as it's too easy to have the fault current for primary faults to increase beyond the bayonet ratings and no one will recognize it.