Ohm's Law

  morddwyd 12:32 06 Jun 12

My electric hedge trimmer keeps blowing its 3 amp fuse.

I've measured the off-load resistance at 23 ohms.

Now my Ohm's Law is a bit rusty, but am I right in thinking that 240 volt across 23 ohms will require more that a 3 amp fuse?

  Quickbeam 12:41 06 Jun 12

Ohm my God... I had a similar problem with the new garage wiring being on trip outs when rewired last month. The mower would run OK on a 5 amp fuse with wet grass, then it started tripping out.

So I just stuck a 10 amp trip in to allow for the start up loading, if that blows, then there's definitely something wrong.

  Woolwell 12:49 06 Jun 12

I hope you have a RCD.

  john bunyan 12:56 06 Jun 12

I am no electrician, but only use 3 amp fuses for lights. I would have thought a 10 amp would be needed, but, as Woolwell says, do use a RCD

  amonra 13:00 06 Jun 12

The 23 ohms is a bit meaningless because it's the impedence that matters to an AC supply. As quickbeam said earlier, stick a bigger fuse in, stand back, switch on and get ready to switch off again. Very often in small machines, the manufacturer fits a radio-interference suppression capacitor across the input switch. These go wrong quite frequently and are easily snipped off to prove a point. It's a case of suck-it-and-see !

  Condom 13:01 06 Jun 12


I was useless at that when I was at school but my B&D has a 13 amp fuse in its pre-fitted plug and although I seldom use it it has never blown either its own fuse or the RCD.

  wiz-king 13:05 06 Jun 12

Ohm's law is not for AC use! I would try a 5A fuse or if you can still read the Wattage on the service plate then Watts/220 = Amps

  interzone55 13:09 06 Jun 12

I always look at the Watt figure on the label, then divide by 230 to get the fuse.

If it's 1000w you'll need a 5amp

  Aitchbee 13:28 06 Jun 12

Sometimes trying a different electrical socket in the house makes a difference...my ancient X-BOX only 'fires up' on one...out of about fifteen. (Fifteen-To-One!)

  morddwyd 14:08 06 Jun 12

I know enough about Ohm's Law (and H&S!) to know that fitting a bigger fuse is an absolute no-no.

If the manufacturer says fit a 3 amp who am I to argue<

Didn't trip the RCD any any of the occasions it's blown.

  Aitchbee 14:20 06 Jun 12

Buy a battery-operated hedge-trimmer...you'll get about twenty minutes trimming per charge.

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