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Posted By Topic: HRC Fuse ratings

Jan 29 2020 10:04

Hi all

I'm unsure of the full meaning of the writing on and HRC fuse.

25M32 gM 550v AC 80kA = amp?
16M20 gM 550v AC 80kA = amp?

Its the cable protection rating i'm after.



Jan 29 2020 10:41

I'm sure someone will correct me if needed, but for your first one, it's 25A continuous, and the 32A is the time/current characteristic which determines its ability to withstand the motor starting current.

You size cable for the LOWER rating.

Jan 29 2020 13:14

correct the first figure is the rating that relates to CCC of the circuit - though I'd put it the other way; you don't select cable for the protection rating, you select cable to carry load current - and then provide it with protection according to the as-installed CCC.

And yes the second figure is the starting current the fuse can withstand, though I'm unsure of the time factor; which I believe varies according to fuse Type (that's the "gM" bit: g for general and M for motor-starting).

Jan 29 2020 19:15

I always thought gG was general and gM was motor, and gM could withstand 6 times starting current

Jan 30 2020 09:00

This link is to a document that provides some good info on fuse ratings & applications:

(Aunty Google has some gold among the dross)

Basically a gG (general) fuse provides both o'l & s/c protection; while on motor circuits the starter provides the o/l protection & the fuse only has to do s/c (& fault) protection; so we use a gM.

Note the reference to type 2 co-ordination. That's where the fuse not only clears the fault, but also protects the downstream equipment (motor, starter, etc. If you don't have Type 2 co-ordination between devices; you may well have to replace the equipment after a fault. eg welded contacts, o/l thermals damaged (therefore no longer reliable); maybe even over-stressed windings.

Also the limited starts-per-hour (DOL) for the standard ratings. With a VFD or soft-start, there's probably no need to use gM fuses.


Jan 30 2020 09:18

This one has time / current data:

Jan 30 2020 12:19

I was told to use fast blow HRCs for semiconductor devices like VSD drives to give the drive some protection. This was after replacing a fried VSD for a DC motor on a large plastic extruder, gS was the utilisation category. There is even gR which is even faster acting.


Jan 30 2020 13:54

Wiring rules only requires us to provide s/c & fault protection to the circuit, and overload for the conductors. plus, separately, overload protection for a motor.

But yes good practice is to go further; so that when the "required" protection operates to clear a fault, the damage caused equipment during the time it takes to operate is minimised.