Neutral Backfeed

[Clitheroe]

I have an idea what is meant by the term 'Backfeed' but I would like it if somebody could try and explain it more so I could understand properly /see if I'm correct.

I looked in a plumbing book before which had a glossary of electrical terms in it and under the term 'Single Pole Switch' it said something like 'A switch that only isolates the Phase conductor only, therefore the danger of 'Backfeed' down the neutral conductor exists'.

Any help in helping to understand the above appreciated.

 

[ban-all-sheds]

I've looked in all my electrical books, and I can't find anything to do with plumbing - I think you should take the same view vis-a-vis electrical advice in plumbing books...

Provided circuits have been properly wired up, there is no danger of "backfeeding" (whatever it means) down the neutral.

If circuits have not been properly wired, and the danger does exist, then the answer is to sort out the wiring, not replace SP switches and isolators with DP ones.

 

[SwindonSpark]

Clitheroe, have a search for the following term: -

"borrowed neutral"

 

[breezer]

the easiest way to expalin "back feed" is "something is going the wrong way"

so for example. (imagine this)

live rail on left neutral rail on right

you connect a lamp between the two

lamp lights, no surprise there

now suppose you put a switch on the live side (as you would)

you turn switch off, but now you meter the voltage across the switch, it will show your mains voltage, this is becasue the neutral is backfeeding through the lamp, take the lamp out, and you will not get this.

sometimes wires can get damaged, and the conducters touch, what then happens is that one circuiy can backfeed through the other, it can cause lots of grief trying to find it.

a similar thing happens with car tail lights.

you have seen some one indicating and breaking and say the reverse light comes on, this is becuse the reverse light is being backfed through the other lights, since they all have one common terminal that shopuld be connected to the chassis, bu the connection is poor or failed

 

[bothyhead]

breezer, is there any chance of explaining this with a diagram pls?

 

[pdcelec]

A common backfeed on a neutral is the landing light. The phase conductor is from the downstairs circuit and the neutral is from the upstairs circuit. When the landing light is switched off, and other upstairs lights are on, the landing light (if incandescent) will give an orange glow (which is the small amount of backfeed from the other lights)

 

[ban-all-sheds]

I hope breezer isn't beavering away with his crayons....

Here's one I did earlier:

//www.diynot.com/forums/viewtopic.php?p=123427#123427

 

[breezer]

I hope breezer isn't beavering away with his crayons....

what, you have crayons? i have to make do with coloured sticky paper

 

[ban-all-sheds]

Tell the truth, breezer - they won't give you crayons any more because you eat them.

 

[breezer]

well i only suck them, they last longer that way

 

[ban-all-sheds]

fnarrr fnarrr

 

[bothyhead]

Thanks for the diagram BAN, very well explained!

 

[Softus]

I've looked in all my electrical books, and I can't find anything to do with plumbing - I think you should take the same view vis-a-vis electrical advice in plumbing books...

I don't. An electrician wouldn't know where to start when replacing an immersion heater element, and yet it would be entirely reasonable for a plumber to do the like-for-like disconnection/reconnection of the element to its electrical supply.

For this reason, it is sensible for a plumber to be aware of the recommendations wrt double-pole switches, and be able to recognise the hazard (see below) that a SP switch represents.

Provided circuits have been properly wired up, there is no danger of "backfeeding" (whatever it means) down the neutral.

I believe you're wrong, if the definition of backfeed is taken to mean that the neutral can potentially (no pun intended) supply a hazardous voltage if the switch (in the above scenario, amongst others), is a SP. Also, where an unswitched FCU has been used, and the fuse removed, this also represents a hazard.

Or am I wrong?

If circuits have not been properly wired, and the danger does exist, then the answer is to sort out the wiring, not replace SP switches and isolators with DP ones.

And this is also where the knowledge is important to the plumber - he can point out the flaw to the customer and recommend that a suitable qualified contracter be engaged.
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lynda, modertor

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