Daisy Chain extension cordk

[Caroline Sullivan]

Hi, first time here,

Can I get some safety advice about plugging 2 x 4 gang power bars into a 2 gang extension cable. I read everywhere that plugging one extension into another is dangerous, but can it be done if the sockets aren't overloaded?

In my office, the central heating is broken and we've were given about 10 electric oil radiators to plug in. Originally each person had 2 monitors and 1 pc plugged into a 4 gang power bar which was then plugged into a single floor socket. There were no spare floor sockets for the heaters so we spaced them out and like this:

(1) 1 electric heater plugged directly into a single floor socket.

(2) 4 monitors and 2 pcs are plugged into 2 x 4 gang bars, plugged into a 1 x 2 gang extension, plugged into one floor socket. This load is about 7.5 amps so well under the 13 amp maximum load - but is it safe to daisy chain the extensions like this? Please advise.

 

[E.Swain]

I'd be more worried about plugging in 10 oil heaters. What wattage are they? And are they all on the same circuit?

 

[Lucien Nunes]

Daisy-chaning extension leads is not inherently dangerous but there are some caveats.

1. Total load - you've thought of this. I would be surprised if your four monitors and two PCs used 7.5A between them. I expect you've used the ratings printed on them which is sensible, however it is very much a worst-case figure.

2. Loop impedance. If you extend beyond even a modest length, where the circuit is not equipped with RCD protection, the resistance of the earth cable becomes high enough that in the event of a fault that gives someone a shock, the fuse or circuit-breaker won't work fast enough to stop the shock in time to be safe. This does not really apply where an RCD is providing protection, as this can deal with faults even when the resistance of the cable is high.

3. Reliability of earthing. Every plug and socket between the appliance and the supply adds another possible point of failure. With ten cables daisy-chained, you have ten places at which the earth may be detached, any one of which will impact the safety at the far end.

Your setup sounds fine, but as above, it sounds like these oil-filled heaters, which are typically 2kW, might overload the installed wiring, depending on how many circuits they are spread between.

 

[Caroline Sullivan]

I'd be more worried about plugging in 10 oil heaters. What wattage are they? And are they all on the same circuit?

Thanks for your reply! We were told by the electrician, who supplied the radiators that the load was fine, the heaters are 2000 watts.

 

[Caroline Sullivan]

Daisy-chaning extension leads is not inherently dangerous but there are some caveats.

1. Total load - you've thought of this. I would be surprised if your four monitors and two PCs used 7.5A between them. I expect you've used the ratings printed on them which is sensible, however it is very much a worst-case figure.

2. Loop impedance. If you extend beyond even a modest length, where the circuit is not equipped with RCD protection, the resistance of the earth cable becomes high enough that in the event of a fault that gives someone a shock, the fuse or circuit-breaker won't work fast enough to stop the shock in time to be safe. This does not really apply where an RCD is providing protection, as this can deal with faults even when the resistance of the cable is high.

3. Reliability of earthing. Every plug and socket between the appliance and the supply adds another possible point of failure. With ten cables daisy-chained, you have ten places at which the earth may be detached, any one of which will impact the safety at the far end.

Your setup sounds fine, but as above, it sounds like these oil-filled heaters, which are typically 2kW, might overload the installed wiring, depending on how many circuits they are spread between.

Thanks, as above, I was told the circuit load is OK. But your advice is very detailed and honestly too technical for me, so I will ask another electrican in to have a look and mention all of this to them to be on the safe side. Thanks, this forum is brilliant, I thought I might have to wait a couple of months to get replies!

 

[E.Swain]

I'm only a trainee but it seems excessive to me to equipt a property with 10 oil filled electric heaters. 2000w heaters would draw 8.7Amps each. If you've got 10 on at one time even if they were spread over different circuits your drawing an enormous amount of current. It doesn't leave a lot of current for other circuits, I'd be very unsure to switch the kettle on. But I am only a trainee, I'm sure a more competent or qualified person could point out if I'm mistaken here.

 

[Lucien Nunes]

We don't know how big the office is.. It could be hundreds of square metres with 50 circuits.

 

[E.Swain]

I knew I would of overlooked something. All I could imagine was a domestic 100A supply fuse with 10x 2Kw heaters.

 

[Deleted member 105166]

Hi Caroline, if you can post a photo of the office distribution board/consumer unit, we may be able to give a more informed answer. For example, if you have a three phase supply, with sockets balanced across the three phases, a good amount of separate circuits and plenty of spare capacity on the board, this may indeed be OK. If sockets or cables start getting warm, then it's not OK, putting things simply.

 

[Caroline Sullivan]

Hi Caroline, if you can post a photo of the office distribution board/consumer unit, we may be able to give a more informed answer. For example, if you have a three phase supply, with sockets balanced across the three phases, a good amount of separate circuits and plenty of spare capacity on the board, this may indeed be OK. If sockets or cables start getting warm, then it's not OK, putting things simply.

Thanks, but we will get the electrician in for this, closing this now and really appreciate all the advice I got here. Good night.

 

[Pete999]

Thanks for your reply! We were told by the electrician, who supplied the radiators that the load was fine, the heaters are 2000 watts.

Not a good idea IMO

 

[rapparee]

Not a good idea IMO

She's closed this. She's going to show her spark this forum..... I'd know how I would react if someone was shoving a phone under my nose second guessing my advice.

Let her at it.

 

[Pete999]

Thanks, but we will get the electrician in for this, closing this now and really appreciate all the advice I got here. Good night.

good night

 

[Pete999]

good night

 

[johnduffell]

2. Loop impedance. If you extend beyond even a modest length, where the circuit is not equipped with RCD protection, the resistance of the earth cable becomes high enough that in the event of a fault that gives someone a shock, the fuse or circuit-breaker won't work fast enough to stop the shock in time to be safe. This does not really apply where an RCD is providing protection, as this can deal with faults even when the resistance of the cable is high.

Is that true? I used to think the same but i calculated once for a 20A type b with a socket right at the limit, followed by a 13A plug fuse and a length of cable, and from memory it came out as tens of metres.. whatever the result was i was surprised anyway.
However i agree with your other points especially as I've watched some recent John Ward videos of extension cables with no cpc whatsoever and it's a bit scary.

 

[Flanders]

Daisy-chaning extension leads is not inherently dangerous but there are some caveats.



2. Loop impedance. If you extend beyond even a modest length, where the circuit is not equipped with RCD protection, the resistance of the earth cable becomes high enough that in the event of a fault that gives someone a shock, the fuse or circuit-breaker won't work fast enough to stop the shock in time to be safe. This does not really apply where an RCD is providing protection, as this can deal with faults even when the resistance of the cable is high.

.

I agree with your other points but I would of thought the 13 amp fuse in the plug top would be good enough to cover the disconnection time . Unless of course the extension lead was realy long .

 

[GBDamo]

Had exactly this situation before Xmas, only fan heaters, blew the 60A fuse for the distribution circuit.

Told them how many they could safely run and was called back two weeks ago as they did the same thing.

Shrug..