Minimum protection upstream of a 100A fuse?
Minimum protection upstream of a 100A fuse?
(OP)
Hi,
A 100A "High Rupture Capacity" fuse is supplying a tariff meter.
The curves of the 100A fuse and an upstream 160A circuit breaker set at 140A, when shown with tolerance/band, are touching at the long term delay section.
The curves of the 100A fuse and the 140A CB, when shown as centreline only, do not touch (fuse completely under circuit breaker).
The question is: what is the minimum circuit breaker required upstream of this fuse to ensure complete protection, and is the centreline trick legit?
Thanks.
A 100A "High Rupture Capacity" fuse is supplying a tariff meter.
The curves of the 100A fuse and an upstream 160A circuit breaker set at 140A, when shown with tolerance/band, are touching at the long term delay section.
The curves of the 100A fuse and the 140A CB, when shown as centreline only, do not touch (fuse completely under circuit breaker).
The question is: what is the minimum circuit breaker required upstream of this fuse to ensure complete protection, and is the centreline trick legit?
Thanks.






RE: Minimum protection upstream of a 100A fuse?
In some cases, it may not be practical to ensure complete coordination and you may decide to live with the possibility of the CB tripping unnecessarily.
RE: Minimum protection upstream of a 100A fuse?
Is this normal? C50 MCB downstream of a 100A fuse and still not ok?
RE: Minimum protection upstream of a 100A fuse?
You will probably find that a Type B MCB will be fine if your load doesn't require the inrush capability of the Type C.
RE: Minimum protection upstream of a 100A fuse?
In my opinion you shouldn't use the centreline only method. You should be checking tolerance bands. It doesn't surprise me that a 100A fuse and 140A breaker upstream aren't discriminating.
As for the MCB under the fuse I'd be really surprised if a 50A MCB discriminated with a 100A fuse upstream. It is not just about checking the time current curves, you need to check manufacturer tables to see what happens in the instantaneous region. MCBs under a fuse tend to have pretty low discrimination values. The problem is that fuses start current limiting at a relatively low current. A 100A BS88 fuse will start current limiting at about 1.3kA, so unless your prospective fault level is below this, you will not be able to achieve full range discrimination regardless of what MCB rating you use under it. It is a trade-off.
You might want to check AS3000. They have some ratios for upstream to downstream ratings that they deem to be OK for discrimination (not that I agree with them). I'm not sure they cover fuse/cb situation though....that got put in the too hard basket.
RE: Minimum protection upstream of a 100A fuse?