"aM" rated fuses..

[apprentice87]

Hi All,

Was looking to buy a switchfuse today. I was looking at one which was perfect in all respects except one, it only takes them poxy 10 x 38 fuses...

What it is for is this; Imagine a length of 6mm SWA protected by a 40A MCB. At the other end of the SWA cable you have to fit a means of isolation, and terminate a 2.5mm flexible cable into it, that feeds a mechanical saw that draws about 15A. [Actually there are TWO saws]

Ok, so the saws, having a motors larger than 1/2 HP or whatever it is, are wired through a proper starter complete with thermal overload protection.

Now, left to myself, i would fit a switchfuse to each saw, with suitably rated gM fuses to give "backup" overload protection [for want of a better term] to the saw motor, and also overcurrent protection for the flexible cable leading to the saw, [both overload, [although the motor starter will already see to this] and fault current too.

Now, a gM fuse would be just perfect, but they do not seem to be available in 10 x 38 fuse sizes, all you can get are "aM" fuses.

These things do not provide overload protection, just fault current protection.. What is the point of that?? In the event of the 2.5mm cable being damaged, or a fault in the wiring of the machine causing "fault current" to flow, as Zs even at the end of the 2.5mm would still be sufficiently low, the 40A MCB at the origin of the SWA would operate anyway..

I was thinking of throwing away the fuse holders and fitting a TP, 16A, "C" curve MCB, which would have the added benefit, that, it would shut off all three phases in the event of overload or whatever, and prevent the numpty saw owners, from leaving the thing to carry on running on the two remaining phases and burning out even faster!!!!

Comments please!!

john..

 

[phil d]

I like the MCB idea,had one of my clients ring up once, motor running slow, a fuse had gone and it was trying to run on 2 phases, luckily no damage but MCB does prevent this.If it were me doing the job that would be my way of doing it, incidentally you can get plug and socket arrangements that combine an isolator and either MCB or RCD.

https://www.cef.co.uk/catalogue/products/1150746-16a-3p-e-415v-ip67-red-interlocked-socket-with-din-rail

 

[steptoe]

I like the MCB idea,had one of my clients ring up once, motor running slow, a fuse had gone and it was trying to run on 2 phases, luckily no damage but MCB does prevent this.If it were me doing the job that would be my way of doing it, incidentally you can get plug and socket arrangements that combine an isolator and either MCB or RCD.

https://www.cef.co.uk/catalogue/products/1150746-16a-3p-e-415v-ip67-red-interlocked-socket-with-din-rail

I fitted one of those recently for a cement silo, cracking job, no messing about with separate enclosures. 

 

[Sidewinder]

Don't get hung up about it an aM with suitable TM overload for the motor is fine.

The aM will take care of SC & large overload of short duration, the motor TM overload on the contactor will do everything else.

It's just that a gM should do both that's all.

 

[apprentice87]

Thanks all!!

Do like them socket things!! could save a lot of messing... What i have do though, is to provide a 3P+N supply to a saw, and also a 230V socket, well two of them actually.

What i am thinking of doing, is you can get fairly cheap rotary switchfuses in quite a spacious metal enclosure. I am planning to do two things:

Firstly, fit a pair of IP44 or IP67 230V sockets to the sides of the enclosure, secondly, install a longer DIN rail in the thing, so i can fit terminal blocks, [i do not like stuffing multiple wires in terminals] and also a 20A RCBO to feed the two sockets.

The cable will enter the enclosure [which is IP65 rated] through a CW gland.

What makes me laugh, is that this is going to be installed indoors, but the building is so ****, that the rain soaks all the electrics, and somehow, even though i did not think it possible, takes out not one but two cutout fuses at a time.... How is this possible, well, 100A fuses in the cutout and 200A ones "protecting" a submain do not help!!! Still, my bit will be water proof

john..