Help with weird AC problem anyone?

[Loafer]

Right all you electronically unchallenged, here's the sitch:

 

Fitted new inverter last month, a cheap (£239) 2kW pure sine generic job, from an eBay Shop with good feedback.

 

Since then a strange thing happens - whenever I run our stereo, our RCD trips after about 20mins. If we reset it, it runs for about 15mins then trips again. The inverter itself doesn't trip, just the RCD in our leccy cupboard.

 

In addition, the 'reverse polarity' light is permanently on in our leccy cupboard, which I believe means the neutral has volts on it relative to Earth.

 

I believe the inverter is a centre-tapped earth, which would explain the reverse polarity light being on. But it doesn't explain to me why it runs everything else, e.g. electric drill, a 1300W Krupps coffee machine, MacBook charger etc, all without a hitch.

 

The stereo amp is a Cambridge Audio domestic job, although small, and has given excellent service for 5 years. It still does, but only for about 20 mins!

 

Any ideas, men (OR women!)?

 

 

PS I thought I had posted this question a few days ago, but it 'disappeared'. Apologies if you've tried to answer this before.

Edited November 12, 2015 by Loafer

[nicknorman]

Do you have some other means of powering the stereo eg access to shore power - through the same RCD?

 

If not, how do you know there isn't a genuine fault with the stereo that is causing the tripping?

 

If the answer to that is "it works fine on shore power through an RCD" then I perhaps there are some filtering components (capacitors) connected to earth within the stereo, and perhaps the "pure sine wave" waveform isn't as pure as might be. This means it contains higher frequency harmonics which find it easier to get through the capacitors to earth, and thus the earth current (ie differential between live and neutral) marginally exceeds the RCD's trip threshold.

 

Not sure how you fix that except to /1 get a better inverter 2/ replace the stereo or 3/ remove the RCD (BAD IDEA!).

 

One other thing to try, put an incandescent mains light, or some other mains resistive load, on at the same time to load the inverter a bit. This can help to improve the waveform, although of course that is only a diagnostic aid not a solution because you won't want a power hungry lamp on all the time you are listening to the radio.

[Guest]

Not saying this is the problem but we used to get strange effects with computers when connected to RCD's. A sudden change of temperature could cause problems (always assumed it was condensation on the panels) and dust build up could do similar. We used to blow them out with compressed air.

 

That wouldn't explian why it only does it with your new inverter of course, but I have often found strange coincidences within electrical faults.

[Loafer]

Do you have some other means of powering the stereo eg access to shore power - through the same RCD?

 

No. All shore or gen power just goes straight to battery charger

 

If not, how do you know there isn't a genuine fault with the stereo that is causing the tripping?

 

Just a feeling. I have no way of knowing other than rigging it direct to generator. I may give that a try tomorrow!

 

If the answer to that is "it works fine on shore power through an RCD" then I perhaps there are some filtering components (capacitors) connected to earth within the stereo, and perhaps the "pure sine wave" waveform isn't as pure as might be. This means it contains higher frequency harmonics which find it easier to get through the capacitors to earth, and thus the earth current (ie differential between live and neutral) marginally exceeds the RCD's trip threshold.

 

Not sure how you fix that except to /1 get a better inverter 2/ replace the stereo or 3/ remove the RCD (BAD IDEA!).

 

We ARE considering replacing our stereo amp with a 12V DC unit, but nothing looks suitable. I don't want just a car stereo, I like my 'sounds'!

 

Thanks Nick anyway. I have only a slim hope of finding a solution!

 

 

Edited November 12, 2015 by Loafer

[cuthound]

I agree with Nick's diagnosis. It's either a faulty stereo or a harmonic problem.

 

Try the stereo amp plugged directly into the mains. If it works OK then it is probably a combination of the amp and inverter generating harmonics and causing the rcd to trip.

[Loafer]

Nice idea - but I have no mains! But I'm noting the harmonics thing. I'll try it with my generator, direct, tomorrow!

[nicknorman]

Nice idea - but I have no mains! But I'm noting the harmonics thing. I'll try it with my generator, direct, tomorrow!

Try it with another load plugged into the inverter at the same time, eg inspection lamp etc preferably with an incandescent bulb.

Edited November 12, 2015 by nicknorman

[ditchcrawler]

centre taped inverter, neutral to earth fault in stereo.

[Guest]

centre taped inverter, neutral to earth fault in stereo.

That used to happen on the electroguard supplies that we had in the school labs. They were centre tap. N-E fault will trip them no problem.

[James T Berk]

Aerial

[Psycloud]

I have a similarly odd problem with a similar generic PSW eBay inverter.

 

We have some 240v LED fairy lights in the boat and when running the inverter they glow very dimly when plugged in but not switched on, so somehow electricity is flowing through the controller despite the plug being turned off.

 

All the other sensitive electronic devices work without issue (apart from a powered speaker which suffers with ground loop hum) but I can't figure out how to stop the LED's lighting up (other than unplugging them).

 

ETA: They do not do this on shorepower or generator

Edited November 13, 2015 by Psycloud

[cuthound]

I have a similarly odd problem with a similar generic PSW eBay inverter.

 

We have some 240v LED fairy lights in the boat and when running the inverter they glow very dimly when plugged in but not switched on, so somehow electricity is flowing through the controller despite the plug being turned off.

 

All the other sensitive electronic devices work without issue (apart from a powered speaker which suffers with ground loop hum) but I can't figure out how to stop the LED's lighting up (other than unplugging them).

 

ETA: They do not do this on shorepower or generator

Modified sine wave inverters generate loads of harmonics, which then flow to earth. This is what is lighting your LEDs and causing the earth loop hum. Unfortunately harmonics are made up of many frequencies which can make them difficult to filter out.

 

I don't think you can do anything to stop them lighting when switched off other than unplugging them or going for the drastic solution of replacing the inverter with a PSW one.

[Chewbacka]

Just thinking aloud - but if there were a capacitor between neutral and earth in your stereo it would be designed as a lowish voltage as neutral and earth should be at near enough the same voltage. However in your case the neutral is 110v AC away from earth, and so it may be leaking too much.

[RLWP]

Aerial

 

Because the aerial is connected to neutral and hence to the boat hull - and the earth and/or neutral of the inverter is also connected to the hull?

 

Richard

[ditchcrawler]

I have a similarly odd problem with a similar generic PSW eBay inverter.

 

We have some 240v LED fairy lights in the boat and when running the inverter they glow very dimly when plugged in but not switched on, so somehow electricity is flowing through the controller despite the plug being turned off.

 

All the other sensitive electronic devices work without issue (apart from a powered speaker which suffers with ground loop hum) but I can't figure out how to stop the LED's lighting up (other than unplugging them).

 

ETA: They do not do this on shorepower or generator

If like the OPs its centre tapped earth they even with the socket switch off you have 110 volts going to the lights via the neutral conductor as switch sockets are normally only switched in the live. Likewise if you delve into anything that is still plugged in you are likely to get a belt even with the switch off.

[Loafer]

Blimey. Some stuff to chew on there.

 

Why on 'earth' does anyone want a centre-tapped earth then, if it causes all sorts of problems? Sounds daft!

Just thinking aloud - but if there were a capacitor between neutral and earth in your stereo it would be designed as a lowish voltage as neutral and earth should be at near enough the same voltage. However in your case the neutral is 110v AC away from earth, and so it may be leaking too much.

 

Doesn't a capacitor ALWAYS leak AC?

 

Sounds like a bit of a bum purchase on the inverter. Does anyone know of a test I can perform to see if I can earth the neutral at the inverter without blowing it up?

[Guest]

Blimey. Some stuff to chew on there.

 

Why on 'earth' does anyone want a centre-tapped earth then, if it causes all sorts of problems? Sounds daft!

Certainly caused us problems in the school where I worked.

[Chewbacka]

Advantage

The max voltage between a 'hot' wire and earth is 110V which is safer than 220V. Very common on construction sites etc.

 

Disadvantages

Need a double pole switch to fully isolate appliances

Needs a double pole circuit breaker so an overload will disconnect both sides of the supply

etc............

[nicknorman]

Why on 'earth' does anyone want a centre-tapped earth then, if it causes all sorts of problems? Sounds daft!

Apart from the above-mentioned, I think it is electronically easier and cheaper to make a centre tapped inverter than a fully floating one. That said, our Travelpower is pretty expensive but is centre-tapped!

[ditchcrawler]

When I worked for Birds Eye foods all the guard interlock system ran on centre tapped 50 volts AC due to it getting hosed down with hi pressure hot water so shock risk was very low at 25 volts to earth. the down side was that an earth fault due to water would sometimes just be enough to hold the relays in when a guard switch was broken not every time but occasionally. We are going back 45 years

[Loafer]

Modified sine wave inverters generate loads of harmonics, which then flow to earth. This is what is lighting your LEDs and causing the earth loop hum. Unfortunately harmonics are made up of many frequencies which can make them difficult to filter out.

 

I don't think you can do anything to stop them lighting when switched off other than unplugging them or going for the drastic solution of replacing the inverter with a PSW one.

 

But Psycloud's IS a PSW one, as is mine!

[Chewbacka]

As it was less than £250 for a 2kW inverter I suspect the word Pure does not mean 100% pure.

It would be interesting to look at the wave form with a scope.

[Loafer]

As it was less than £250 for a 2kW inverter I suspect the word Pure does not mean 100% pure.

It would be interesting to look at the wave form with a scope.

 

 

That's what I thought.

 

I'm somehow convinced that if I could bond the negative to the earth, the problem would go away.

 

How about an isolation transformer? I could surely bond the negative and earth from the output of that, and use it to connect to the boat?

 

Could that be an answer to getting my hifi back?

[Paul G2]

I have a similarly odd problem with a similar generic PSW eBay inverter.

 

We have some 240v LED fairy lights in the boat and when running the inverter they glow very dimly when plugged in but not switched on, so somehow electricity is flowing through the controller despite the plug being turned off.

 

All the other sensitive electronic devices work without issue (apart from a powered speaker which suffers with ground loop hum) but I can't figure out how to stop the LED's lighting up (other than unplugging them).

 

ETA: They do not do this on shorepower or generator

 

Are the lights switched with a regular wall-mount light switch? If so, depending on how the lights/switch are wired, you might be able to use a DPDT switch and switch both the hot and neutral lines. That should solve your problem

[cuthound]

But Psycloud's IS a PSW one, as is mine!

Whoops. Depends on the components used to generate the sine wave as to how pure the sine wave is. Better quality ones use IBGTs, whilst cheaper ones use SCRs (thyristors). The less perfect the sine wave, the more the harmonics genersted before any !oad is connected.