Charles Gregory wrote:
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.
"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).
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.
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.
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.
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"...
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.
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)?
Any ideas - or any recommendations of where to buy 0.8A fuses - I'm going to
need some more!
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
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