back feed explained

[vibetime]

sorry if this has been covered , but i have researched and got many different views.
so, what is a back feed and how do they happen ?

 

[WDMDL]

An unwanted interconnection between circuits.
Classic example upstairs and downstairs lights - and the landing light arrangement.

These days, we run T&E to landing switch from light, then 3&E between switches.

Olden days, they picked up a L from the hall light switch (as part of 2 gang switch)
Then ran T&E between switches
Then ran a single L to landing light, and a single N from landing light to nearest upstairs light.

Joe Spark comes to work on upstairs lights, locks off the MCB. Prove, test, prove...

He has got a bunch of wires in his hand, feeding them through the base of new light fitting.
But someone switches the landing light on (it doesn't come on) and.... many of those wires in Joe's hand are now live as he has broken the neutral and 60 Ohms worth of cold filament in the landing light won't limit the current through his body!
It's not unheard of to find unwanted interconnections between other circuits... I've found them between cooker circuits and ring finals!

Simon.

 

[ian.settle1]

An unwanted interconnection between circuits.
Classic example upstairs and downstairs lights - and the landing light arrangement.

These days, we run T&E to landing switch from light, then 3&E between switches.

Olden days, they picked up a L from the hall light switch (as part of 2 gang switch)
Then ran T&E between switches
Then ran a single L to landing light, and a single N from landing light to nearest upstairs light.

Joe Spark comes to work on upstairs lights, locks off the MCB. Prove, test, prove...

He has got a bunch of wires in his hand, feeding them through the base of new light fitting.
But someone switches the landing light on (it doesn't come on) and.... many of those wires in Joe's hand are now live as he has broken the neutral and 60 Ohms worth of cold filament in the landing light won't limit the current through his body!
It's not unheard of to find unwanted interconnections between other circuits... I've found them between cooker circuits and ring finals!

Simon.

This is incorrect as it is also a statement used where the main supply has been disconnected and a temporary supply is connected to keep equipment running eg When you have a HV supply you may need to work on the transformer and to keep the LV equipment running you would connect a generator to the LV side and this is known as back feeding.

 

[Engineer54]

This is incorrect as it is also a statement used where the main supply has been disconnected and a temporary supply is connected to keep equipment running eg When you have a HV supply you may need to work on the transformer and to keep the LV equipment running you would connect a generator to the LV side and this is known as back feeding.

That's not correct either!!

Back feeding, using your example would be for the generator supplying the transformer to achieve MV at the primary side of the transformer!! Maybe a better example is a PV installation, where the inverter is supplying power ''back'' into the DNO LV network...

There are many installations where a stand-by generator is part of the general supply installation, that kicks in on loss of prime power. That would be classified as an alternate power source, it's not back-feeding anything...

 

[vibetime]

Can someone clarify it , if it was in a normal heating circuit for example ?
people have different views on this as described above.
I understand borrowed neutrals.

 

[vibetime]

Anyone else got any views ?

 

[Engineer54]

Can someone clarify it , if it was in a normal heating circuit for example ?
people have different views on this as described above.
I understand borrowed neutrals.

No idea what your trying to say here!!

 

[vibetime]

Heard it can happen in heating circuits sometimes n just wanted some clarity in what is what

 

[alarm man]

well that depends on what you mean by back feed,ie you isolate one circuit which some how has a feed from another circuit onto that circuit.not common neither is it uncommon

 

[mechelec]

You say in a normal heating circuit. Do you mean the orange wire (HW call) from the three port motorised valve which serves two functions with power going either way depending on valve position which without diagrams I cant explain here. If so just search google

 

[Welchyboy]

i allways thought backfeed was where, if you have isolated a circuit and it contains capacitors or such-like and they are holding a charge, in some instances they can discharge back down the circuit (eg if you make contact with the isolated conductor/s remaking a path for the current)

As for a heating circuit??? dunno?