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Power At Lighting Wire When Switch Is Off ?


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#1 nobby478

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Posted 16 April 2016 - 04:38 PM

I found this site on the net whilst trying to understand a wiring problem I have..my first post here.


We've been in our house 12 years and inherited a brick-built shed with a waterproof switch outside that operated a PIR controlled outdoor carriage lamp. A strange set-up, I know

The PIR got rainwater in it and I decided to re-wire it without the PIR. I spent a few hours on the lamp, then went to the twin and earth wire that supplied it , cut the corroded ends away and received a small shock.

I checked the switch was turned off and cut the wire again....and got a spark!

So I took the switch apart and removed the switched wire. I knew it was the switched as it also had the wire that goes to an LED.


I still got a spark when I trimmed the wire again.

I gave up and put the kettle on.



On reflection, the switch had a red supply wire and a black switched wire, but the wire at the lamp was normal twin and earth, so there must be a junction somewhere, but why the power at the lamp end? It all worked correctly before the PIR as damaged. Car wiring I understand.....house ring mains not so good...


Thanks for any guidance.......

Edited by joiner, 17 April 2016 - 07:25 AM.
Typo


#2 ProDave

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Posted 16 April 2016 - 04:43 PM

There's probably a permanent L feed to the PIR sensor so the light comes on when activated, and the switch is an overide.

You should ALWAYS isolate at the consumer unit and properly test for dead, not just turn off a light switch and hope.

Edited by joiner, 17 April 2016 - 07:25 AM.


#3 nobby478

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Posted 16 April 2016 - 05:16 PM

Thanks for your reply.
Unfortunately I don't know which fuse covers the shed or where it draws its power from. There is no permanent L to a PIR now as I took it all out when re-wiring the lamp....the lamp and its PIR were switched, you'll recall.

I'm able to type OK now ....some mis-match before. The shed was set up as granny flat...she had to switch PR light on to get to the house toilet. the previous owner didn't like his mother-in-law...no hot water or heating........

Is it of note that we have no earth circuit, just ground spikes?

Edited by joiner, 17 April 2016 - 07:26 AM.


#4 jsharris

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Posted 16 April 2016 - 05:20 PM

As Dave says, never rely on a single pole lightswitch, as it's possible that a numpty has wired the switch to break the neutral, rather than the live.

There are two ways of wiring light switches, the older way was to run the lighting ring to the light fitting (loop in light), with a cable from there down to the switch. This meant that you always had live and neutral at the light fitting somewhere, irrespective of the light switch position. The way it worked was to take the live on the lighting ring (so two wires joined) to a live to the switch (a single length of T&E), what would normally be the neutral in the switch T&E becomes the switched live feed to the light fitting, and should be sleeved or taped with brown or red tape to show that it's now a switched live. The neutral in the lighting ring connects to the neutral of the light fitting, and the earths are all connected.

This system is a complete pain, because although it means that there is just a single cable leading to each light switch, it means that there are several terminations at the light fitting.

The newer (and better) system is to loop in the switch, where the lighting ring runs from one light switch to the next, with a length of T&E running to the switched light fittings. This has loads of advantages; it's far easier to make all the connections at the switch, rather than at the light, the light fitting shouldn't have a live cable if the switch is off (but never trust a light switch) and the wiring is usually a lot easy to understand, as at the switch you have the lighting ring in, the lighting ring out and the cable running from the switch to the lights.

Edited by jsharris, 16 April 2016 - 05:21 PM.


#5 Nickfromwales

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Posted 16 April 2016 - 05:25 PM

Hi.
If you don't know which circuit / fuse serves the circuit in question then you turn them ALL off by flipping the main switch in the fuseboard OFF ;).
At the moment you seem intent on electrocuting yourself so I'd advise getting an electrician around ASAP and stand down with your own efforts / mis-haps.
You only die once, and that's not a Bond film !
Regards, Nick.

#6 nobby478

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Posted 16 April 2016 - 10:01 PM

Thank-you fellers...unfortunately I live in an area where electricians rarely visit and charge £150 to ring your doorbell if they do.....

It's annoying that it worked before but I shall have to accept it isn't viable to make it work again.

How annoying that I can diagnose a fuel injection fault and understand it's wiring diagram but I can't get the hang of a ring main...my bad.

I'm not going to set it up again if I can't work out why the lamp wire had current with the switch set to off........




.

Edited by nobby478, 16 April 2016 - 10:02 PM.


#7 cjard

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Posted 17 April 2016 - 05:35 AM

Because, as someone else has already commented, there is no guarantee that a switch breaks a live wire. If it were in a neutral wire, the light would still go on and off, but would be "live" all the time just waiting for its "neutral" to come along and complete the circuit

PIR lights have to have a permanent live feed to power the sensor. THe sensor is a switch like any other, think of it like a person who forever stands next to a simple light switch. When they see you, they switch on the switch. When you go away they switch of. If you have an override switch (you don't really expand on what that means) it either verrides so the light is always on, or so that it is always off. If always off, it will cut the power to the light fitting, could be in either the live or neutral feed to the unit and is in series with the fitting. If overrides to always on, then it is in parallel with the PIR switch, so either the PIR switch being connected (when it sees you) or the manual switch being flipped gives the power somewhere to go and light the lamp.

If it were a circuit diagram:


(I gave up on using the CODE tags on this forum - it has weird, stupid bugs and doesn't present the text as entered. See attached file)

Now imagine some nob swapped the L and N over..

Attached Files

  • Attached File  cd.txt   287bytes   7 downloads

Edited by cjard, 17 April 2016 - 05:38 AM.


#8 nobby478

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Posted 17 April 2016 - 07:23 AM

The PIR did indeed have a live feed...straight from the positive terminal on the connector block on the back of the lamp to the PIR circuit board. So as the light was controlled by a switch it wasn't permanent, but a switched PIR live feed.

It partly seems the trick with this BB is to move your cursor clear of the message window.

I do take your point about the possibility of the neutral being switched, not the live.......the numpty who did it didn't earth the lamp either. He really didn't like the MIL.....



I'll give it one more try with the fusebox turned off, before wiring it through a 3 pin plug!

Thanks all.

Edited by joiner, 17 April 2016 - 07:29 AM.
"trough" to "through"


#9 nobby478

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Posted 17 April 2016 - 09:38 AM

Well, I risked the wrath of her indoors and turned the house power off. The lamp is up and running...earthed now.

House vendor had indeed wired the switch through the negative line as some of you suspected.

Thank-you.


How on earth do yu put up wth this sites typng issues?

#10 SteamyTea

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Posted 17 April 2016 - 09:47 AM

Go and upset your better half and turn the power off again, take all the fuses out and work out which each one does.
Then write it down and put it in the fuse box.

#11 jsharris

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Posted 17 April 2016 - 10:04 AM

View Postnobby478, on 17 April 2016 - 09:38 AM, said:

How on earth do yu put up wth this sites typng issues?

What problems are you getting?

We know that the code tags don't seem to format included text as expected, and the search facility isn't great, but I'm not aware of any typing issues that are a consequence of the forum.

I am aware that some browsers on certain operating systems can have a typing lag. For me I used to get it on Firefox on an Android table (fixed by switching to a simpler browser) and also Firefox on Linux Mint 17.2 and Linux Mint 17.3. I haven't yet played around with the latter two, but the lag (you can type text and it doesn't appear for a couple of seconds) seems specific to Firefox and Linux-based systems, as far as I've been able to tell.

I'm currently typing this using Firefox on an old Windows 7 machine and it never lags at all.

Others have reported similar oddities, so my guess is that it's something to do with the way the browser works, rather than something specific to the forum.

FWIW, Chromium running on Linux Mint seems to get around the typing lag problem. What are you using to look at the forum? There's a chance that someone here has already found a fix.

Edited to add:

I found these diagrams on the web illustrating the loop in switch arrangement:

Posted Image



and the loop in light arrangement:

Posted Image

Edited by jsharris, 17 April 2016 - 10:20 AM.