one for the pro's to debate...

[ColJack]

following from the "spot the error" thread...

can you have just a single or double socket wired in 2.5mm² from a 32A breaker?

I know you can have one as a spur from a ring and that spur can be from the source of the ring ( ie straight from the breaker ) , but does that then mean that you can wire JUST a single or double straight from the breaker..
you could argue that it's a spur from a theoretical ring of zero length...

 

[Spark123]

Yes, you can. Overload protection is provided by the plugtop fuse and fault current by the circuit protective device.

 

[ColJack]

so on that basis then, i can put just one single or one double socket straight off the service fuse then? the rating of the breaker has no effect on the cable size only the max load?

 

[Spark123]

I doubt the service fuse will provide fault protection for the cable, given the CPC is 1.5mm and the let through energy I^2t of the protective device will be a bit higher than that of a 30A device.
Of course it depends on the service fuse size/type and the leccy board might not be impressed with you for bypassing the meter.

 

[martinxxxxxx]

No, it is a radial so should have a 20A breaker for 2.5mm cable.

 

[Spark123]

As long as the circuit is protected against overload and fault currents then a 30A protective device can be used, just like installing a spur from a RFC directly from the consumer unit.

 

[GaryMo]

No, it is a radial so should have a 20A breaker for 2.5mm cable.

Because that's a standard circuit arrangement?

Yes, most of us would do that but in some circumstances there's nothing wrong with a 32A MCB supplying a twin or single socket or a FCU via 2.5mm2 conductors.

If you believe it to be incorrect then what regulation do you think it contradicts?

 

[ColJack]

yes but that's a specific exception from the rule..

and with a ring main, presumably the mcb is going to be loaded anyway so the breaker will react faster to a problem..

 

[ColJack]

No, it is a radial so should have a 20A breaker for 2.5mm cable.

Because that's a standard circuit arrangement?

Yes, most of us would do that but in some circumstances there's nothing wrong with a 32A MCB supplying a twin or single socket or a FCU via 2.5mm2 conductors.

If you believe it to be incorrect then what regulation do you think it contradicts?

that would be 433.1.1...

433.1.1 The operating characteristics of a device protecting a conductor against overload shall satisfy the following conditions:

(i) The rated current or current setting of the protective device (In) is not less than the design current (Ib) of the circuit, and

(ii) the rated current or current setting of the protective device (In) does not exceed the lowest of the current carrying capacities (Iz) of any of the conductors of the circuit, and

(iii) the current (I2) causing effective operation of the protective device does not exceed 1.45 times the lowest of the current carrying capacities (Iz) of any of the conductors of the circuit.

specifically part (ii) of it..

 

[GaryMo]

Have a read of 433.2.2, specifically (i)

 

[Spark123]

and with a ring main, presumably the mcb is going to be loaded anyway so the breaker will react faster to a problem..

What if the cable wasn't being loaded and someone put a nail through the cable?

 

[Lectrician]

We had this with the NIC inspection once.

I noted down that two hand driers where both fed from a single 32amp MCB (two lengths of 2.5mm to DP isolators, both from the same MCB).

The area engineer agreed, and asked how I would rectify it if we where going to. (inspecting one of our PIR's only).

I said we would split them into two circuits, which is likely how they where where first fixed anyway.

He replied with "or change the DP isolators for FCU's". And indeed you could Not great, and a little against the grain, but compliant.

You need to provide fault protection aswell as overload protection, and this can be done with a single device at the origin, or the fault protection only at the origin and the overload protection can be downstream. There is cases where the device at the origin cannot provide either of these, and in these cases there are three main points that your circuit must adhere to. ANYONE?

 

[mfarrow]

Have a read of 433.2.2, specifically (i)

I don't think that would apply as a double socket creates a branch. Plus the protective device would itself be an outlet for current-carrying equipment, so I'm not sure how that would play.

More likely it is allowed by 433.3.1 (ii).

Diversity has been allowed for in the design of the 1363 double socket outlet which allows them to be rated for a current draw through both sockets summing to 20A. Therefore as long as the installation method of the cable does not reduce its current-carrying capacity below 20A, then I would say it is allowed.

 

[GaryMo]

Have a read of 433.2.2, specifically (i)

I don't think that would apply as a double socket creates a branch. Plus the protective device would itself be an outlet for current-carrying equipment, so I'm not sure how that would play.

More likely it is allowed by 433.3.1 (ii).

The maximum loading a double socket can take is 26A (2x13A), probably less as most are rated lower. 2.5mm2 t&e cable, clipped direct is rated at 27A.

I don't see a problem with 433.2.2 (i)

 

[ColJack]

433.2.2 specifically says

433.2.2 The device protecting a conductor against overload may be installed along the run of that conductor in the part of the run between the point where a change occurs ( in cross-sectional area, method of installation, type of cable or conductor, or in environmental conditions ) and the position of the protective device has neither branch circuits nor outlets for connection of current-using equipment and fulfills at least one of the following conditions:

(i) It is protected against fault current in accordance with the requirements stated in Section 434
(ii) It's length does not exceed 3 m, it is installed in such a manner as to reduce the risk of fault to a minimum, and it is installed in such a manner as to reduce to a minimum the risk of fire or danger to persons ( see also Regulation 434.2.1 )

please note the highlighted section..
would you consider a single or double socket an "outlet for connection of current-using equipment"?

your follow on argument will likely be that the plugs all contain fuses...
but it doesn't specify that and that's not entirely true either..
there are special "red" plugs that contain no fuse.. while these are not in general use they are fitted to some protable machines that have large current demands for a short duration, such as portable x-ray machines..

while the example of the hand dryers would satisfy 433.2.2 in some way if the faut current requirements were met, this is only because they have permanent connections and not outlets...