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Posted By Topic: HRC vs MCB for plasma cutter

Oct 24 2019 21:34

Hi all

I've recently installed a plasma cutter for a client and two months down the track he has had a few jobs where the cut patten has a high amount of starts on the sheet. The MCB is tripping out on overload. Im picking its the inrush warming up the bi metallic strips. Looking into replacing the MCB with HRC's but can't seem to find anything regarding the high frequency starts on the inrush. Any ideas?

Oct 24 2019 22:54

Motor rated HRC fuses or D curve MCBs may help.

Oct 24 2019 23:02

Sorry forgot to say already have a D curve MCB in place

Oct 25 2019 07:56

What is the running current compared to the breaker?
How does that compare to the inrush current?
It might be a case of having to increase the supply cable to the machine which will enable you to increase the size of the breaker. Might be costly but would be a fairly simple solution.

Oct 25 2019 09:23

HRCs are not all the same either.

you can get "motor rated" fuse links to cope with this sort of problem. the rating will be given as eg "32 M 40" or "40 M 80", first figure being the rating for constant current(to protect cable) and second the motor-starting rating for short-duration.

There is still a thermal build-up from multiple starts over short period, but it will be somewhat different than for an mcb.

Another option might be a "motor circuit-breaker" with adjustable overload setting; but (without checking) I'd assume these will be operating on a D-curve or very similar.

So if multi-start operation is likely to be a common mocde of operation, upgrading the cable may be the best option. For just the occasional time, could be left as an operational matter.
The cutter itself will have a duty cycle governing how many starts it should be allowed in a given time.