Monitor Speaker tripping the mains circuits (UK) insulation test ok, what next? - Page 1
Forum section: electronics / repair

author: Reprobyte
author_rank: Regular Contributor
timestamp: July 10, 2024, 05:36:44 pm
content: Hello, purchased a used pair of JBL 705p monitor speakers, and I have found that 1 of them has tripped circuits 4 times.1st time plugging in, tripped upstair circuit.2nd time switching off switch next to IEC cable, tripped whole house.3rd and 4th time on downstairs circuits, tripped when the extension that its plugged into was turned on at the wall with the rest of the setup.This is over the course of a couple weeks.I got out the Megger BM80 and tested the insulation and it’s at 370MegaOhms both poles to ground.The device works fine when on, seems to be in a state of powering on or powering off. I have not yet opened the unit up as I am unsure if its the unit or something on my house, advice needed.Any more tests I can do here, only really had a similar issue with a 1500VA step down transformer in the past doing the same thing.All advice is greatly appreciated.

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: July 10, 2024, 06:30:01 pm
content: Hi Reprobyte,what kind of breakers do you have (Amperage and characteristic)?What RCDI (whole house on one RCDI is a bad idea, imho)?370 Meg Iso seems reasonable.Good luck

author: Reprobyte
author_rank: Regular Contributor
timestamp: July 10, 2024, 06:34:18 pm
content: Hope this helps, these are the ones that tripped, I don't know if that explains. House electrics are not my strong point, I can wire the sockets but never messsed with the main box.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: July 10, 2024, 06:58:25 pm
content: It sounds to me that you have a fault with the entire house installation that need checking out by a good competent electrician who understands RCD’s and RCBO’s, NOT one of the part P brigade.G Edmonds

author: Gyro
author_rank: Super Contributor
timestamp: July 10, 2024, 07:23:50 pm
content: You appear to have highlighted the first floor lighting MCB. I hope you mean the ground floor sockets MCB next to it!My first thought is inrush current. MCBs can get tired with age, especially if tripped multiple times. When breakers are in close proximity, it is possible for the magnetic field from one to affect the adjacent one on a heavy current surge. The first floor sockets MCB is right next to the RCD that tripped. It could be a magnetic interaction rather than an earth leakage fault, especially as the other RCD, feeding the ground floor sockets MCB didn't trip.It's curious that one speaker trips the circuit and the other one doesn't (?). It would be worth comparing the innards to see if the NTC thermistor has been bypassed on the tripping one - assuming that the PSUs are SMPS.Other thoughts, Have you got anything plugged into the signal inputs? If so, disconnect and try again. It would be worth checking for continuity (I wouldn't risk HV insulation test yet) between the signal ground and the primary side.

author: Reprobyte
author_rank: Regular Contributor
timestamp: July 10, 2024, 08:22:33 pm
content: Sorry I did mean that, I selected the wrong one in the box my mistake there it was not the lighting it was the sockets my bad there.Yes they do use switching power as far as I am aware.It's not something that happens every time, its more of an every so often thing, but its done it with signal connected and disconnected.Thanks I will do some testing on the continuity of ground, didnt think of that, appreciate the advice here.

author: themadhippy
author_rank: Super Contributor
timestamp: July 10, 2024, 08:44:11 pm
content: QuoteAs its an overload device and not the rcd thats tripping i'd lean more towards a live-neutral issue

author: Reprobyte
author_rank: Regular Contributor
timestamp: July 10, 2024, 10:15:01 pm
content: Quote from: themadhippy on July 10, 2024, 08:44:11 pmThanks for the advice, what is the best way for me to work out if that's the case here?

author: shakalnokturn
author_rank: Super Contributor
timestamp: July 11, 2024, 12:12:03 am
content: To second themadhippy, this shouldn't be an earth fault, i'd check if the offending speaker has a moulded input EMI filter, I've seen those have intermittent shorts on a couple of occasions.If supply is not SMPS the transformer could be suspect too, generally check for pinched or stripped wires.Tie your continuity tester to the mains inlet, give the speaker a few jerks / slaps. If no luck open it an wriggle things around, apply a little pressure here and there.

author: Gyro
author_rank: Super Contributor
timestamp: July 11, 2024, 07:52:18 am
content: It was probably a mistake for me to include the afterthought of the signal ground continuity test (simple and quick though it is). I agree with the others, it is a L - N surge current or intermittent short fault, as I explained, I think the RCD trip was due to it being right next to the MCB that tripped.Presumably the speakers have some internal mains supply fuse. If this isn't blowing (it clearly isn't) then any intermittent short would be upstream of that - but I don't think it is, intermittent shorts rapidly turn into hard faults. They will definitely be visible on inspection if [they areit is] tripping a 32A MCB*.Again I think you need to do an internal comparison. As I mentioned earlier, my gut feeling is that something is going on with the (SMPS) input surge limiting NTC thermistor. Maybe it failed on that speaker and the previous owner linked it out. Most SMPS faults (transformer etc) will result in a 'catastrophic' component failure rather than an intermittent.I'm sorry, but I don't see any alternative to unscrewing the rear panels and inspecting. If you post photos of both speaker internals then we have a better chance of spotting something.Edit:*Being in the UK, you also have a mains fuse in the plug.