Double socket keeps failing

[JohnW2]

.... Also, all the houses I've lived in have been new builds, so haven't had botched wiring

You are seemingly a 'man of faith'

Kind Regards, John

 

[EFLImpudence]

Thanks, I understand what you are saying about Fused Spurs and spurs. It has always seemed a little strange calling an FCU a fused spur, but as you say this tends to be what the laymen knows them as (including myself).

You now know better.

Regarding a spur that is fused (connected to an FCU), this is the first time I have come across this, but that's probably because I don't really do much with electrics.

Ok, but how else could you have a fused spur (the cable)?

Out of interest, is there a "better" way (I guess that would mean safer way) to wire up these sockets?

I don't think so.

Of course, an FCU for ONE socket is not really necessary but that's nothing to do with safety.

It depends what you are meaning by 'sockets'.

At counter level there are three sockets next to each other:
Fused Spur Switch 1 | Fused Spur Switch 2 | Double Socket

Even the sellers do not call FCUs sockets.

 

[Milleniumaire]

Ok, but how else could you have a fused spur (the cable)?

I meant I hadn't ever considered that a spur socket would need to be fused i.e. connected to a FCU. I assumed that a single cable would be run from another socket to the new spur or a terminal block would be used to connect the new spur upto the ring circuit. In either case there is no FCU.

I assume it depends on the load that may occur at the spur? As you say, the FCU is required to protect the cable that the spur is connected to.

It depends what you are meaning by 'sockets'.
Even the sellers do not call FCUs sockets.

I appreciate this, it was me being lazy with my typing.

 

[JohnW2]

I meant I hadn't ever considered that a spur socket would need to be fused i.e. connected to a FCU. I assumed that a single cable would be run from another socket to the new spur or a terminal block would be used to connect the new spur upto the ring circuit. In either case there is no FCU.

That is true, provided that there is only one socket (an 'unfused spur'). If more than one socket (or a socket plus 'other things') is supplied by a spur cable from a ring (and 2.5mm² cable is being used for the spur), then there has to be an FCU to 'limit' the total current draw of all the spurred sockets etc. to 13A.

Kind Regards, John

 

[EFLImpudence]

Unless, of course, you use a cable suitable for the 32A - 4mm².

 

[Notch7]

That is true, provided that there is only one socket (an 'unfused spur'). If more than one socket (or a socket plus 'other things') is supplied by a spur cable from a ring (and 2.5mm² cable is being used for the spur), then there has to be an FCU to 'limit' the total current draw of all the spurred sockets etc. to 13A.

Kind Regards, John

I suppose a spur with a 2.5mm cable going to a double socket could potentially overload the cable if a 3kw heater was plugged into both of the sockets.....

Whereas as EFLI says if it was connected by a 4mm cable then that cable would have a limit of 32A and the MCB at the CU then protects the cable from overload.

(just interested in seeing if I understand the logic -not trying to say Im correct!)

 

[JohnW2]

Unless, of course, you use a cable suitable for the 32A - 4mm².

Quite so - as I wrote ...

If more than one socket (or a socket plus 'other things') is supplied by a spur cable from a ring (and 2.5mm² cable is being used for the spur), then there has to be an FCU ...

Kind Regards, John

 

[EFLImpudence]

I suppose a spur with a 2.5mm cable going to a double socket could potentially overload the cable if a 3kw heater was plugged into both of the sockets.....

Not really; the cable can take 27A. That's why a double socket is allowed.
That only one single socket is allowed does not really make sense.
Two singles are better than one double.

Whereas as EFLI says if it was connected by a 4mm cable then that cable would have a limit of 32A and the MCB at the CU then protects the cable from overload.

4mm² can take 37A, but it is more than the 32A of the MCB, so more than one double socket would be compliant.

 

[Milleniumaire]

Unless, of course, you use a cable suitable for the 32A - 4mm².

Do you mean a 4mm cable from the consumer unit i.e. for the ring circuit, not just 4mm cable for the spur part?

 

[JohnW2]

I suppose a spur with a 2.5mm cable going to a double socket could potentially overload the cable if a 3kw heater was plugged into both of the sockets.....

3kW is about 13A. 2 x 13A = 26A. The current carrying capacity of 2.5mm² cable (installed 'Method C, as most commonly) is 27A, hence, since 27 is more than 26, no problem

Whereas as EFLI says if it was connected by a 4mm cable then that cable would have a limit of 32A and the MCB at the CU then protects the cable from overload.

Indeed - if one uses 4mm² cable for the spur (again, Method C), then, as far as the spur cable is concerned, the spur can supply as many sockets as one wants (with a 32A MCB). However, very few people, including many electricians, really think of that possibility - they have merely learned that "an unfused spur must not supply more than one socket", without having given any thought to the matter of cable size.

If one puts several sockets on a 4mm² unfused spur from a ring (hence all connected to the ring at the same point), there is (if that point is close to one end of the ring) a risk that one part of the 2.5mm²) ring cable might get overloaded - but that's a different matter.

Kind Regards, John

 

[EFLImpudence]

Do you mean a 4mm cable from the consumer unit i.e. for the ring circuit, not just 4mm cable for the spur part?

No, just for the spur cable. It can the handle as much current as the ring.

Rings are an anomoly since the introduction of MCBs.
However, the regulations for rings make no distinction between MCBs and the rewirable fuses which they still include.
These fuses require larger cable than their Amperage rating.

 

[Milleniumaire]

So the 2.5mm ring cable isn't likely to be affected if the load on the spur(s) is at the limit of the 4mm spur cable?

Is this because the 2.5mm cable is on a ring i.e. both ends of the cable are connected to the consumer unit? Or is this because the MCB would trip and protect the 2.5mm ring cable?

 

[EFLImpudence]

So the 2.5mm ring cable isn't likely to be affected if the load on the spur(s) is at the limit of the 4mm spur cable?

Sort of. It won't be on the limit of the 4mm² (37A) but the limit of the MCB (32A).

Is this because the 2.5mm cable is on a ring i.e. both ends of the cable are connected to the consumer unit? Or is this because the MCB would trip and protect the 2.5mm ring cable?

Both. With the proviso, as John said, that the full load should not be near either end of the ring.