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Posted By Topic: Discrimination between HRC and MCB

Jul 06 2018 19:33

Where will I find the characteristic curve for a 63A HRC pole fuse?

Jul 06 2018 19:57

Your best bet is to go to the manufacturers web site. Of course you will need to know the details of the fuse to select the correct device from the vast number that are available.

I had a quick look at the Eaton web site and they have literally hundreds of fuse types and there are curves available for all of them.

This is a link to a page on the Eaton site that has curves for fuses that resemble the traditional clip in fuse for pole type fuse holders.

And here is a link to offset bolted tag type fuses as used in 63A pillar type fuse holder.

But you must get the data for the actual fuse used.

Jul 06 2018 20:14

Thanks. I\'ve got a renovation that will exceed 63A max demand. So I was trying to work out whether a 63A MCB (MD by limitation) would discriminate with the pole fuse.

I\'ve checked a few 63A gG HRC datasheets and based on what I\'ve found, it can\'t work. I feel like i\'m missing something.

Jul 06 2018 20:43

The MCB discriminates at low and high currents but the curves cross over in the middle.

So in practical terms an overload up to around 2I should trip the MCB but approaching the transition to instantaneous trip the HRC should go first. (c) requires discrimination over the entire instantaneous curve.

Jul 09 2018 22:37

Have another read of the 1st para of the clause, which gives the purpose of discrimination. You\'ll find that there is no requirement for discrimination between upstream & downstream devices on th4e same length of cable, only between devices for outgoing circuits and an upstream device.
With your set-up, either device operating will remove supply from the entire installation, so discrimination provides no advantage.

Jul 10 2018 06:53

AlecK thanks, that makes sense. I expect excess demand (versus a fault) will be in the slow part of the curve and trip the MCB first. So the consumer retains the convenience of turning a few things off and resetting without a call to the power company.

Jul 11 2018 23:30

Didnt think we were Allowed to discriminate these days...

Jul 12 2018 17:30

BrianW I think age is the only discrimination allowed these days. Often times it seems to be encouraged.

Jul 12 2018 17:42

In USA they seem to be discriminating based on politics:\"I don\'t like who you work for so you can\'t eat in my restaurant\".
For NZ that isn\'t allowed; the things we can\'t discriminate about are listed in law (Human Rights Act ). There\'s nothing there about fuses or circuit breakers, so we\'re allowed to discriminate. Indiscriminately.