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Power amp causing fuses to trip, only when it is turned off!

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Power amp causing fuses to trip, only when it is turned off!

Postby GiggingAddict » Tue Jul 18, 2017 3:08 pm

Hi all

I have had a problem with a PA system in my bands rehearsal studio for the last few days that we are at a loss to understand, hopefully someone may have some suggestions. :) We have a PA set up in our rehearsal studios, and about a year ago we had the electrics fully checked over and serviced, got a certificate from the electrician to say that everything was all safe, and we had no problems until a couple of days ago.

At the end of the rehearsal session we turned off the power amplifier in the way that we usually do - turn the faders down, turn the volume of the power amplifier down all the way, turn off the power amplifier, and leave the mixing desk switched on. However, when we switched the power amplifier OFF, it tripped the fuse in the fuse box on that circuit. We turned the PA system off and reset the fuse, and then turned on the power amplifier for about 30 seconds and switched it off, tried this a few times and about once every 3 times turning off the power amplifier would cause the fuse to trip out, but not every time, just once in ever 3 times or so. However, there's no problem at all when it is switched on, of when we are playing, but only when the power amplifier is switched off. :thumbdown:

So we went through the usual steps to try to work out what the problem was, and this was when it got a little strange. There are two different ring circuits in the room, so we switched the PA to the other circuit, and the problem switched to the new circuit, with the fuse for this other circuit now tripping out. So, we think, the problem is with the PA system. However, there are other studios in the same building so we go to them to ask if we ca borrow their power amplifier to see if we can pinpoint the problem, and when we try to use their power amplifier, the same problem persists. Then we get a third power amplifier, which is another band's back up power amplifier, and again the same problem persists on both of the circuits - no problems when turning on the power amplifier, or when playing through it, but when we switch if off, the problem persists. We switched the speakers around to ones that were from another studios that are only 4 months old and have had no problems, and still the problem persists on both circuits and through different amplifiers.

Outside of the room there are no problems with the power amplifiers, which are 2 different Behringer EP2000s and a Behringer EP4000, they're reliable, and no fuses being tripped outside of the room. We tried to turn on and off the guitar and bass amplifiers in the room, to see if the same thing happened, and we could not get the fuses to trip by turning them on and off in the same manner, even though the bass amplifier is a 400 watt one. The electrician that we used last time is on holiday at the moment and we have booked for him to come in when he returns, but that will be in about 14 days time, which is not ideal. :thumbdown: The problem is on two different circuits, each of which has their own 32amp fuse, both of which were checked over in October last year and given a clean bill of health, and we never had any problems until a few days ago. We can't work out why the problem would suddenly happen with multiple different power amplifiers, on two different circuits at the same time.

We ran the equipment straight into the electric socket, and then through a 4 way Surge Protector plug, didn't make any difference. We tested the plugs with our plug tester, seems to be no problem there. The weird thing is that the power amplifiers, which seem to be the cause of the problems, don't cause any problems outside of the room in other rehearsal studios, so it's hard to pinpoint them as the blame, and yet the problem happens over two different circuits, so it doesn't seem to be a problem with a particular plug or a particular fuse.

The only bit of equipment that is the same in all of the tests is the mixing desk, but we don't switch that on or off, so we didn't think that that was the problem. Also, the cables from the power amplifier to the speakers are the same. But apart from that, all of the factors are being changed: different circuits (2 of them), different power amplifiers (3 of them), different plugs on those circuits, different kettle leads, but we do have the same mixing desk and speakon cables from the power amplifier to the speakers, but as the problem only happens when we turn the power amplifier off, we didn't think that that was the problem. The fuse box is also a common factor, but it is different fuses within it that are tripping.

All in all it is very confusing to us!! :roll: :( We're usually very thorough when it comes to these kind of things, as you can never be too careful, but we're at a loss as to what the problem could be, so any suggestions that anyone could offer would be gratefully and warmly appreciated.

Thanks!! :)
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Re: Power amp causing fuses to trip, only when it is turned off!

Postby James Perrett » Tue Jul 18, 2017 4:01 pm

When you say it tripped the fuse I assume that you mean that it tripped a circuit breaker. It might be worth checking whether this is just a circuit breaker (MCB) or whether it is an RCD (residual current device) which will trip when there is a current imbalance between live and neutral. It might help if you could post a picture of the circuit breaker.

I'm also assuming that you are in the UK - if not could you let us know where you are in the world as wiring practices differ from country to country.
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Re: Power amp causing fuses to trip, only when it is turned off!

Postby Hugh Robjohns » Tue Jul 18, 2017 4:32 pm

I think James is on the right lines here -- part of your confusion at this is the constant referencing to fuses 'tripping' -- I doubt this is a current overload problem at all. Far more likely to be a current imbalance problem that is tripping the RCD element of the circuit breaker.

So... the question is why are you getting a current imbalance when switching the power amps off? And given that the same thing happens on different circuits and with different amps, I suspect it's going to be something to do with a ground loop.

Try disconnecting all of the audio inputs to the amp before turning off, and see if that makes a difference.

It's quite possible that the combined earth leakage from all the gear on the power ring mains is just nudging the trip threshold, and turning off the power amp somehow sets it over the edge (possibly by removing it's own contribution to the earth leakage current which may well have a different phase relationship to other currents).

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Re: Power amp causing fuses to trip, only when it is turned off!

Postby Folderol » Tue Jul 18, 2017 5:06 pm

Very much agree this sounds like earth leakage. It might be worth checking if this happens with the mixer unplugged from the mains. You might be accusing the wrong bit of kit.

What I find curious is that the fault seems to have occurred comparatively recently, some time after wiring changes were made. This makes me wonder if something with filter caps on the mains has 'lost' one, slightly unbalancing the leakage current. As a point of interest filters will always unbalance the leakage to some degree as the cap going neutral to earth will have a much lower voltage across it than the one going live to earth. However, there is frequently a bigger one going live to neutral - just to confuse things further.
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Re: Power amp causing fuses to trip, only when it is turned off!

Postby GiggingAddict » Thu Jul 20, 2017 3:08 am

Firstly, thanks so much for the replies, it's really appreciated to have the help! :thumbup: :clap: :D

Yes, sorry, I should have said that I'm based in the UK. It is two different BG 20A Single Pole B Type MCBs that keeps on tripping, we have 2 of them on the two circuits that are in the room, and they both keep on tripping. (We also have a part of the room sectioned off into a smaller office that is coming from the same fusebox, but as of yet we have not had any problems from the MCB's that are powering this sectioned off office.)

Here is a link to a photo of the fusebox in question. The MCB marked "Fridge, Microwave" is feeding the circuit that is in the office, and the other two B20 fuses are powering the two circuits in our rehearsal studio. It is these B20 MCB's that keep on tripping. (I think that "tripping" is the correct term, apologies if it is not.)

https://ibb.co/erQA9k

We are not in the studio until Friday, but I will try to go in tomorrow to do the recommendations that you all kindly suggested to me. :thumbup: :) Hopefully it will help us to get to the source of the problem, so that we can then get our electrician in to fix the problem. I'll do all of those suggestions, and then report back. Yes, it is strange how the problem has suddenly reared it's head, we've not had many problems until now, apart from one or two other things that were quite small.

Again, thanks so much for the help, it's all really appreciated!! If anyone has any other suggestions, they will all be gratefully appreciated, so that we can hopefully cure the problem without haemorrhaging too much money on it. ;) :thumbup: :clap:
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Re: Power amp causing fuses to trip, only when it is turned off!

Postby ef37a » Thu Jul 20, 2017 5:42 am

If you can do it safely, check the incoming mains voltage.

In Blackstar's first premises we had odd breaker tripping with combinations of variacs. The mains was found to be tapped to 254V!

It is possible that some heavy consumer has gone out of business locally? In another part of our town a big Co-op store has closed. I bet mains volts have increased a bit round there since everything else is small shops and residential.

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Re: Power amp causing fuses to trip, only when it is turned off!

Postby Folderol » Thu Jul 20, 2017 7:49 am

Hmm, that knocks out (sorry) the earth leakage idea! Must be an overcurrent somewhere. It now sounds more like an insulation breakdown, triggered by the spike generated from the switch-off. Is there any sign of even slightly damaged mains cables anywhere? it's not likely to be a dodgy breaker or installed wiring because you said that if you move to the other circuit the fault moves with the kit.
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Re: Power amp causing fuses to trip, only when it is turned off!

Postby Wonks » Thu Jul 20, 2017 10:30 am

I suspect either failed/faulty insulation or possibly a corroded switch in the amp to be the issue. You often get a short arcing period between the switch contacts when switching off something capable of drawing a reasonable current, and if there's faulty or damaged insulation (or possible a loose spade connector), the arc might also be flashing over to neutral or ground and creating a large current draw for a very short period, and so tripping the MCB.
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Re: Power amp causing fuses to trip, only when it is turned off!

Postby Folderol » Thu Jul 20, 2017 1:13 pm

Ha! Good one Wonks!

And if it's a two pole mains switch, a broken internal insulator between live and neutral would be a good possibility.
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Re: Power amp causing fuses to trip, only when it is turned off!

Postby Hugh Robjohns » Thu Jul 20, 2017 1:31 pm

Er... but this tripping problem happens with other amplifiers, and it doesn't happen with the original amps in different rooms in the same building.

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Re: Power amp causing fuses to trip, only when it is turned off!

Postby Folderol » Thu Jul 20, 2017 1:51 pm

Bugrit! Missed that {cough} little {cough} detail :frown:

Oooo kaaay.

What is the current rating for the MCBs powering this room, and what is the current rating for the ones in other rooms? That might give a clue. Also what is the class?

There should be a letter in front of the current. For domestic kit I would expect mostly the following
B6 for lighting
B32 for ring mains.
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Re: Power amp causing fuses to trip, only when it is turned off!

Postby Hugh Robjohns » Thu Jul 20, 2017 1:59 pm

On the image posted of the consumer unit, three circuits are marked 'sockets'.

One has a 32A breaker, and the other two are 20A breakers.

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Re: Power amp causing fuses to trip, only when it is turned off!

Postby Folderol » Thu Jul 20, 2017 2:55 pm

Ok, just had another read through from the start. Still can't display that image for some reason.

I don't know if it's significant, but a 20A breaker screams just one thing to me. It's not a ring it's star wired and I'll bet it's wired with 2.5mm cable (which just happens to be rated at 22A when partially enclosed).

Now this is arguably a 'good thing' for a studio, but personally I'd have gone for 6mm and a 32A breaker. Apart from anything else, in theory a double socket could draw a total of 26A and still be in spec.

When the fault occurs, is there always other kit on that supply, or does it still do it with nothing but the amp plugged in? ... and by that I mean straight into the wall, even with no socket strips plugged in.
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Re: Power amp causing fuses to trip, only when it is turned off!

Postby GiggingAddict » Wed Sep 13, 2017 4:45 am

Hello everyone!

I know that this is an extremely late reply, but I have literally just now remembered that I forgot to reply to this - at about the time that this was all going on I had two family members being taken very ill, so I passed it off to another couple of members of the band, giving them the link to this thread, and they didn't have the log in details, so it didn't get replied to, with everything happening it all got forgotten about. :headbang:

Anyway, as an update and to provide closure, we managed to get a qualified electrician in who found the advice given was really useful. Everything was all checked over and is all up to code, it turned out that for a few days , (as far as I could understand) the electricity coming into the building that we are based in was coming in at a higher voltage, and that many other people in the building were having the same problems as us. Some work was done along with UK Networks, who are the distribution network operator for electricity covering South East England, and it turns out that there was a fault well down the line that we were picking up on our fuse box. They carried out the work that was needed, and since then everything has been all fine, no problems at all, they disappeared as quickly as they raised their heads. :thumbup: This is all from memory, by the way, they explained it to me at the time so I might have got the odd phrase incorrect, sorry if that happened, I'm trying my best to get all of the details. The main thing is that we've got everything all signed off, and 2 different people say that it's up to code. We'll still keep an eye out for things though, to make sure that any problems that come up get seen quickly.

Anyway, I just remembered that I had forgotten to thank people for their help and felt awful for that, so I thought it better late than never. It's been a very stressful time recently, especially personally, so it's been great that people have been kind enough to give their advice on what could have been causing the problem. It really did help to get us focused on what the problem could be, we have now invested in extra equipment that will help us work out what the problem could be in the future, everything has had a bit safety check and is all in order, and so all is right with the world!! :thumbup:

Thanks again, everyone, it's really nice and humbling too to have had your help with it all, it means a lot to us. You are all a credit to the internet, and these boards in particular. I'm now off to have a look at other threads on the forum, in case there is anything that we can help other people with, to pass the good karma on! :)
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Re: Power amp causing fuses to trip, only when it is turned off!

Postby blinddrew » Wed Sep 13, 2017 7:52 am

Glad it's been sorted, and always good to get things properly signed off! :)
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Re: Power amp causing fuses to trip, only when it is turned off!

Postby Hugh Robjohns » Wed Sep 13, 2017 8:22 am

Yes, thanks for the update, and glad it has been sorted. It is easy, sometimes, to forget that some problems can be caused in the delivery chain, too!

Sorry to hear about your family woes. Hope things improve soon.

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