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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> A few stray volts https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1088202382 Message started by Dave on Jun 25^th, 2004, 11:26pm

Title: A few stray volts
Post by Dave on Jun 25^th, 2004, 11:26pm

Here's one I should know the answer to, but can't figure out:

In a lighting circuit (wired as a loop circuit) there are 6 to 8 volts present across the lamp terminals when the switch is in the off position. Should this be so, and where does it come from?

Title: Re: A few stray volts
Post by plugwash on Jun 25^th, 2004, 11:36pm

is there a bulb in or not

getting such things with no load connected is rairly normal due to induction (digital multimeters have a very high imput impedence)

if you are getting it with a bulb in it really does not seem right to me

could be that someone has put earth into the third terminal of a 2 way switch..........

Title: Re: A few stray volts
Post by Dave on Jun 26^th, 2004, 7:57am

No. I noticed it with no load connected. I was using a digital meter and the voltage I noticed did float about a bit, so I guess you are right about it being due to the input impedance of the meter.

There is no two-way switching on the circuit and I had already checked through all of the connections.

Thanks for the reply.

Title: Re: A few stray volts
Post by rabbit_rabbit on Jun 26^th, 2004, 8:24am

Physically impossible is switched off but often revealed by 'meters' still meters are leaning towards the pessimistic side of things which I guess is right, but wrong - if ya know waht I mean. Yes high input impedance.

RR

Title: Re: A few stray volts
Post by HandyMac on Jun 26^th, 2004, 8:25am

Having a few volts present on a lamp socket seems perfectly okay to me - if there is no lamp in the socket.

But those "volts" will be accompanied by negligible "amps" should you put a load across the circuit (e.g. a lamp). Because DVM's are high impedance input (tens of megohms is typical) they draw f-all current when put on a circuit. Not zero, but very low.

Andrew

Title: Re: A few stray volts
Post by rabbit_rabbit on Jun 26^th, 2004, 3:47pm

Sort the bastard out if in doubt should be all of some 5 nanoamps available - don't forget to remove the short before turning on!

That reminds me - when I was a school (a LONG time ago) in the chemistry lab we used to put a 6d piece in the light bayonet fitting and shove the bulb back in place and sat in the dark. When the teacher came in and turned the light on there was a hell of a bang and we would all fall about laughing.

RR

Title: Re: A few stray volts
Post by The_Trician on Jun 26^th, 2004, 8:10pm

Nice one RR - We used to do that too! Also had a prewired 13A plug L-N-E prelinked which we used to use in a local record shop on a Saturday afternnon if they were playing any crap disco music when we went in!

Plug in to a handy skt and 'bang'! the music stops!

Just goes to show that a little knowledge is a dangerous thing when you are a teenage electrical apprentice!

Ah...Those were the 70's!!

TT

Title: Re: A few stray volts
Post by HandyMac on Jun 26^th, 2004, 8:38pm

I can see it now.....

SS is up before the beak for running an online forum where advice is being given out to DIY sparkies that they should try shoving coins into light fittings and shorted 13A plugs into sockets. As the judge puts on his cap to pass sentence:

"Young man, you have been found guilty of the offence for which you were brought in for. You may say a few words before being taken to the gallows....."

Andrew

Title: Re: A few stray volts
Post by plugwash on Jun 26^th, 2004, 9:28pm

he wasn't advising to try it he was just saying what he did

Title: Re: A few stray volts
Post by Dave on Jun 26^th, 2004, 10:01pm

This one has really strayed off-subject! However, the various stunts pulled by people in their teens did remind me of the one we used at college. Wire a 240V 1Mf capacitor across the L and N terminals inside a 13A plug top, stick into a socket to charge it, then toss it across the room to some poor unsuspecting individual, who got a pretty nasty belt when he caught it.

Health & Safety? - Never heard of it.

Title: Re: A few stray volts
Post by plugwash on Jun 26^th, 2004, 10:28pm

ofc the size of the shock would depend on where it was in the cycle when you cut it from the supply

Title: Re: A few stray volts
Post by L.Spark on Jun 26^th, 2004, 11:34pm

makes it sound like some weapon of war, rules have changed for capacitors, they have to self discharge now, but thats only if in a circuit ::)

Title: Re: A few stray volts
Post by plugwash on Jun 26^th, 2004, 11:47pm

i guess the have to self discharge is only if they are being changed to over a certain voltage?

in the worst case though that cap in plug trick would be damn nasty

Title: Re: A few stray volts
Post by Lectrician on Jun 27^th, 2004, 10:47am

Charge the cap with your meggar - full 240, 500, 1000v then :-)

Title: Re: A few stray volts
Post by L.Spark on Jun 27^th, 2004, 11:12pm

only discharge when isolated, so as they dont store the charge, otherwise its the good old discharge resistor job to ensure its safe to handle.