Switched FCU noise

[Andromedus]

Hi guys,

I have a switched fused connection unit in the living room - the load was intended to be connected to an electric fire, although I have instead connected the load to a 6-way trailing socket. I also relocated the fcu - no major changes, I just unwired it, moved the box a few inches, and reconnected. It all works fine, but the odd thing is that sometimes when I switch the fcu on I can hear a quiet, high pitch hissing noise which lasts for a few seconds. It's difficult to tell exactly where the sound is coming from, but it seems to be coming either from the fcu itself, or from the cables slightly higher up in the wall connected to it. I have unplugged all electrical appliances from the trailing socket so I'm pretty sure it isn't the sound of an appliance springing into life, and I can't think of any way in which the fcu could be being overloaded. I'm slightly worried about the noise - I can't smell any burning smell or anything like that, but I have a niggling doubt that the whole thing might suddenly burst into flames.

I wasn't aware of anything mechanical inside the fcu which could make a noise - I thought it was basically just a connector box.

Any ideas?

Thanks!

 

[pjcomp]

You could be hearing a bit of arcing across the wires and terminals of the FCU. Check the terminals are fully tightened down and not loose, which would give rise to the arcing. Alternatively, you hit a gas pipe when you moved it ....

pj

 

[bernardgreen]

How many wires were there in the FCU ?

If it was on a ring main, ( two Live wires and two Neutrals ) then a loose connection could create an ark with noise as current flows from wire to wire across the loose connection and on to the next socket. The vibration as the switch moves could dislodge the loose connection enough to start the arc and the sizzle......

 

[Andromedus]

Thanks guys - definitely didn't hit a gas pipe as a) the house didn't explode and b) we're not on gas!

There's just one cable going into 'feed' (ie. one live, one neutral and one earth) and one cable going out of 'load' - so I'm guessing that means it's a spur rather than on the ring main.

I'll double check the terminals are all tightened up, although I'm pretty sure they are.

You can't get any arcing where the cables are insulated, right? It's just that unless it's my imagination, it sounds like the sound is coming from higher up inside the wall than the fcu, where the feed and load cables run. I don't really want to rip the wall out to investigate!

If there is arcing, what's the worst case scenario - could that cause a fire? As the switch is fused, and it's also protected by an RCD, shouldn't it all cut out before that happens?

 

[JohnW2]

There's just one cable going into 'feed' (ie. one live, one neutral and one earth) and one cable going out of 'load' - so I'm guessing that means it's a spur rather than on the ring main. ... I'll double check the terminals are all tightened up, although I'm pretty sure they are.

If that's the case, and if you still hear this noise when there are no loads plugged into the extension, then there isn't really anything to arc. - and ...

... it sounds like the sound is coming from higher up inside the wall than the fcu, where the feed and load cables run. I don't really want to rip the wall out to investigate!

Again, if there are no loads, hence no current to flow through the FCU, in electrical terms, switching on or off the FCU should not have any effect on anything going on in upstream cables.

If you're sure about this noise, and if the problem is continuing, I think you'll really have to get an electrician to look at it. There are certainly some tests which could be done without/before needing to think about digging into the wall.

Kind Regards, John

 

[Andromedus]

Thanks. I've just realised that the trailing socket has a small red light on it which comes on when it's on, so I guess there actually was a load. It also has it's own switch, so I'll turn that off and try again for the noise.

When I was moving the fcu, and hence the cable that runs up the wall to ceiling level, I did have to chisel away at the plasterboard and some of the coving at the top of the wall, so I suppose it's possible that the vibration caused by turning the fcu switch is just dislodging some dried plaster chippings or something which make a noise as they fall down the back of the plasterboard. I have to say, it sounded more like electrical sparks than falling debris, but who knows.

If it persists, I'll call an electrician. Thanks for the advice.

 

[JohnW2]

Thanks. I've just realised that the trailing socket has a small red light on it which comes on when it's on, so I guess there actually was a load. It also has it's own switch, so I'll turn that off and try again for the noise.

That little red neon won't represent a significant load, but those things can sometimes make a little noise, so possibly that's what you're hearing. Switching off the trailing socket will presumably turn that light off, so would be a useful test.

... so I suppose it's possible that the vibration caused by turning the fcu switch is just dislodging some dried plaster chippings or something which make a noise as they fall down the back of the plasterboard. I have to say, it sounded more like electrical sparks than falling debris, but who knows.

I suppose that's not impossible.

If it persists, I'll call an electrician. Thanks for the advice.

You're welcome. Let us know the outcome!

Kind Regards, John

 

[flyingsparks]

Does the switch have a light on it (a neon)? Possibly it's that you can hear.

Edit: too slow should refresh before posting!!

 

[JohnW2]

Edit: too slow should refresh before posting!!

It's good to see someone other than me being the culprit

Kind Regards, John