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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> Got zapped off hifi - earthing? https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1335617833 Message started by lemaiz on Apr 28^th, 2012, 1:57pm

Title: Got zapped off hifi - earthing?
Post by lemaiz on Apr 28^th, 2012, 1:57pm

Hi folks, electrical novice here, be nice!

I've got a couple of hifi separates under the telly and in the process of hooking up another box my arm brushed against the case of the amp/receiver and it gave me a shock. Not a big one, more of a 'buzz' I spose. So I plugged it all in and ignored it for a while, like a proper amateur.

But I thought I should check it out. Got kids running around now. I got the multimeter out and put the com on the adjacent radiator pipe and checked the amp case, the screws on the sub, the external earth on the mains filter box - all show +/- ~1.5V.

What's going wrong? Something to do with earth? I haven't separately earthed anything - I figured the earth on the mains plug covers that, doesn't it?

TIA

Title: Re: Got zapped off hifi - earthing?
Post by Lectrician on Apr 28^th, 2012, 7:38pm

Best to get the earthing checked.

If the building has lost it's earth, then the radiator (assuming copper pipework?) would be at the same potential as the sockets earth, and so your meter would not show a voltage difference.

Can you take a picture of your consumer unit and metering?

Can you measure the voltage between live and earth?

Title: Re: Got zapped off hifi - earthing?
Post by lemaiz on Apr 28^th, 2012, 9:33pm

[gallery]lemaiz/1335644939.jpg[/gallery]
[gallery]lemaiz/1335644940.jpg[/gallery]

Is this what you were looking for? The CU was replaced last year.

Yeah, copper piping all over. Check the voltage between live and earth? How do I do that? Somehow jabbing the pins of the multimeter into a live socket feels somehow... wrong...

Title: Re: Got zapped off hifi - earthing?
Post by Lectrician on Apr 29^th, 2012, 9:45am

To be honest, that all looks perfectly fine, a very tidy job (shame about the hand written legend for the circuits!).

Earthing looks fine, but that doesn't mean it is not faulty, although would have expected whoever changed the CU to have fully tested.

A multimeter can be set measure AC volts, and you can then test from live to earth with the socket loosened and pulled forward, loosen the socket with the power off.

Are you using some dated, homemade or dodgy looking extensions for the HiFi?

Title: Re: Got zapped off hifi - earthing?
Post by CWatters on Apr 29^th, 2012, 6:38pm

Not all electrical equipment is designed to be earthed. Some HiFi equipment is double insulated to help eliminate hum due to earth loops.

Are there any symbols on the amplifier? For example...

Title: Re: Got zapped off hifi - earthing?
Post by lemaiz on Apr 29^th, 2012, 10:55pm

Thanks for the help folks. I did a little nosing around after the comments.

Firstly I thought, it's an amp, it's all about power, how could it not have a ground? How wrong I was.

[gallery]lemaiz/1335735382.jpg[/gallery]

There are the little concentric squares, clear as day near the top. Then here's the posh Isotek 6-way. Not homemade or cheap...

[gallery]lemaiz/1335735383.jpg[/gallery]

It clearly says it must be earthed - but I've never done anything with the Ground on the left. I get around +/- 2V DC (reading flicks around) between that Ground and the radiator but if I switch to AC on the multimeter it suddenly reads about 20V and then quickly drops to stabilise around 4V.

I'm surprised that the Isotek says it must be earthed, I would've thought that plugging it in would automatically earth it through the plug. But then if there was some potential on the earth...

So... should I always have linked that green Ground to the radiator (or some other true ground)? The brief manual doesn't explicitly say to. If the Isotek wasn't properly grounded, would you expect some potential on it?

Lectrician, when you said extensions, what did you mean? Sorry haven't had the stones to play with the mains yet, I'll take a look tomorrow!

Title: Re: Got zapped off hifi - earthing?
Post by CWatters on Apr 29^th, 2012, 11:31pm

The Isotek probably _is_ earthed via the mains plug. The instruction that says it "must be earthed" is probably trying to tell you that you must plug it into an earthed outlet socket.

I would give Isotexk a call. I don't think the earth stud on the Isotexk is intended to be used to earth the Isotek. My guess is it's provided as an earth point for use by other equipment - possibly the chassis or pick up arm of a turntable.

I can't be 100% certain but I'm not sure you actually have a problem.

Title: Re: Got zapped off hifi - earthing?
Post by CWatters on Apr 29^th, 2012, 11:56pm

More reading...

http://www.eevblog.com/forum/general-chat/earthing-rant/

Basically it wouldn't be uncommon to find half the mains voltage on the chassis.

Title: Re: Got zapped off hifi - earthing?
Post by Lectrician on Apr 30^th, 2012, 7:18am

Double insulated kit can have some voltage present on the chassis through some internal filtering. If you reverse the polarity on some much older double insulated kit you can end up with a substantial voltage present.

It is predominantly seen with TV's and their aerial connections.

You may find your amp actually does have an earth terminal too, and connecting this to the earth connection on your power strip would bring the chassis of the amp down to earth potential. This is where earth loop type hums may then become a problem.

Title: Re: Got zapped off hifi - earthing?
Post by londonman on Apr 30^th, 2012, 6:51pm

Frequently in high end stuff you will see a signal earth separate from mains earth. Avoids 'ticky-hum'

Are we sure it's not simply static?

Title: Re: Got zapped off hifi - earthing?
Post by lemaiz on Apr 30^th, 2012, 9:36pm

So it's quite normal to have a few volts on the amp case? So reaching into the back to adjust a cable gives you a shock every time... That can't be ok. Would it be worth connecting the slightly live ground on the Isotek to the radiator, or would that just electrify my whole heating system?!

I will give Isotek a call tomorrow, see what they reckon!