Low Voltage Light Fuse Keeps Blowing!

[Charles Gregory]

Hi,

I've got a light fitting with 3 directional low-voltage lights. The input is the standard 240V lighting circuit via the normal centre-of-ceiling cable. The incoming cable just screws into a set of terminal blocks.

There is a fuse holder on the light fitting marked nearby as .8 A MAX which seems to be on the output of the +ve cable of the transformer.

The problem is that the fuse keeps blowing. I'm replacing the fuse with ones marked as 8 mA fast-blow - then I turn on the light and it seems to work, but on the 2nd or 3rd turn of the main light switch it blows again.

The 240 V RCD for that lighting circuit isn't tripping so I guess that means that there isn't anything wrong with the 240 V side. I've traced all the low-voltage cables and I can't see any visual signs of damage. I've removed the transformer and had a look at it from the outside and again there doesn't seem to be any visable damage.

It was working just fine - now it's not and we've not touched it.

The last test I did with the last spare fuse I had was to remove the low-voltage light fittings (there's two connectors you can break to remove the light fittings and bulbs altogether) - but even without a load on the transformer the fuse blew straight away.

Any ideas - or any recommendations of where to buy 0.8A fuses - I'm going to need some more!

Thanks Charles

[larry]

should that not be 8A if it is on the outgoing side.

larry

[Mike Harrison]

I assume you mean 800mA. I'd think you should be using anti-surge (marked T) fuses.

[Stefek Zaba]

"Marked as .8A"? 0.8A at 12V is barely 10W (9.6) - no way is that the right rating. If this is really on the LV side, then 8A is massively more likely, being 96W-worth so allowing 3 x 25W bulbs and a bit more for lower resistance at switch-on (and being a sort-of standard value).

Now you're really 'aving a larf with the fuse ratings. You won't find any such thing as an 8mA fuse. 8A yes, not 8mA.

Watch us drift further into fantasy land. Whaddya mean, "RCD" - it's inappropriate, and rare, for lighting circuits to be on an "RCD" - a current-balance earth-leakage trip, with a "push to test" button, a rating on it like "30mA" (relating not to total current through it, but max allowable imbalance between current-out-on-L and current-back-on-N). You probably mean an MCB, a miniature circuit breaker - sort of a "resettable fuse", which for a lighting circuit is likeliest to be a 6A rated device. And not having an MCB pop (or indeed an RCD, or an RCBO which combines the two functions) isn't a definitive "clean bill of health"...

Now that suggests a real fault, but isn't at all consistent with "works for one or two switch-ons then blows" reported in your second para. Maybe you let the two sides of the LV output touch when the connectors were flapping loose (or, if one side is referenced to earth - unlikely but not impossible - let the other one touch some metalwork)?

It may be knacked, and you may not know what you're doing well enough to diagnose or fix. Sorry, but there it is. Got a meter? And enough knowledge not to be truly dangerous when using it?

Stefek

[Lee]

Need more information - like what wattage are the bulbs? Sure this fuse is not on the mains input? In any case, do not exceed the fuse value stated on the holder. Obviously.

Is this an electronic or wound transformer btw? If electronic and you are not exceeding the stated wattage in regard to the bulbs, then it's surely a faulty transformer...

Lee

[Lee]

Doh, typed that without thinking about it! It could be a bulb (or even possibly a switch) arcing as well...

Lee

[Dave Plowman (News)]

Well, 0.8 Amp is 800 mA, and you'd not want fast blow for this application anyway. Anti-surge would be more like it.

If it really is 0.8 amp this must be the mains input - 0.8 amp at 12 volts is about 10 watts.

[Charles Gregory]

Thanks to everyone that replied, but special thanks to Dave and Mike who hit the nail on the head.

I got some Anti-Surge 800mA fuses (a typo on my original post!) rather than the fast-blow variety and it seems to work fine now. (My local white goods retailer should change it's name to Dick Turpin at £1.25 per fuse!)

Another observation about the Anti-surge fuses is that as well as being marked T they seem to have a large bobble on the fuse wire itself inside the cartridge.

Thanks again, Charles

[Dave Plowman (News)]

Indeed. Maplin - not known for being cheap, and also with expensive High Street shops charge 1.79. For 10.