single 2.5mm wire in a 32A MCB

Just opened the CU at work, as we need to add another circuit for some A/C in the server room.

There is a 32A breaker marked as Spare with a single 2.5mm wire inserted (So it isn't spare then I assume!) Now, one part of me says this is wrong, it should be a 20A MCB as it must be a radial, but if this were a ring circuit, then I could spur off of it, so the spur would be protected with a 32A breaker.

The installation has passed safety inspections, should it have?

Sparks...

Reply to
Sparks
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On Tue, 21 Feb 2006 16:17:04 -0000 someone who may be "Sparks" wrote this:-

Do you know where this cable goes? Is the MCB on or off?

Correct.

Non-fused spurs can only serve a single or double socket, or one fixed appliance. Incidentally the Wiring Regulations are silent on a spur serving one fixed appliance and one socket side by side, which is essentially the same as a double socket. As a result of this the current that can be drawn down the spur is limited. However, the

30/32A protective device doesn't protect the spur cable properly. Spurs are a balance of risks, as indeed is any other form of wiring. If houses were bursting into flames regularly because of problems with spurs there would no doubt be a change. There was a change many moons ago, because at one time spurs could feed two single sockets (which meant somebody might think they were on the ring).

On a radial circuit any number of sockets could be wired up. Thus the size of the protective must be smaller to protect the cable.

That rather depends on what was inspected. If the consumer unit was inspected then no.

Reply to
David Hansen

MCB is on, but I am waiting until everyone goes home before I turn it off (Although, it would probably be quicker to turn it off and see who shouts!

I will find out what was tested then!

Thanks!

Sparks...

Reply to
Sparks

The OSG states that a non-fused spur feeds only one single or one twin or one permanently connected equipment

As a result of this the

It should protect against short circiuts on a correctly designed circuit.

Adam

Reply to
ARWadsworth

Time to buy a clamp meter?

There is a very useful new type which we use at work in our machine room. It can measure (to a reasonable approximation) the current flowing in a cable with the clamp around the entire cable, rather than just a load carrying core as the "old" type could only do by design.

Needs to be told the cable type under test, presumably does some clever trickery with the fact that the magnetic field isn't zero in close proximity to parts of the cable surface, despite having a balanced flow and return of current overall.

I'll find out the make tomorrow if you mail me (addy above valid).

Extremely useful for measuring circuit loads in and out of UPS's as well as total load on circuits back to the DB and to answer such questions as "how many of *those* servers + disk arrays dare we put in this rack.

Cheers

Tim

Reply to
Tim S

The 32A MCB provides suitable fault current protection for 2.5mm² T&E cable, and the BS1362 fuse in the plug or FCU provides the overload protection for the 2.5mm² T&E. The cable is properly protected, but the fault current and overload current protection have been separated. This is permitted by the regs previding the fault current protection is at the beginning of the circuit, and the overload current protection is before any branches, which it is in the case of a spur.

Reply to
Andrew Gabriel

Can you expand on that please Andrew for the hard of thinking like myself.

Are you saying that 30/32A protection is fine for a circuit run from a single piece of 2.5mm T & E if it only consists of sockets linked to each other in a bus configuration or with just one device on each spur?

Or are you referring to spurs on rings (which is what *might* be being about above, but wasn't the op's original question.

Not being funny here, just trying to understand what you are saying.

Reply to
cpvh

The difference is the time for which the current flows, which affects the total heating effect on the cable.

  1. Fault current - typically a short circuit - will cause a huge current to flow; the 30/32A protection will clear this fairly quickly before damage to the cable results. Caveats: (a) in some areas (particularly London) the maximum prospective short-circuit current is extraordinarily high because of the density of the supply network. An ordinary MCB might not break such a current quickly enough and indeed might not be able to break such a current at all (contacts weld together). (b) rewireable fuses blow slower than cartridge fuses or MCBs, so cables protected by rewireables have to be downrated by IIRC 20%
  2. Overload current - typically too many appliances - will cause moderate overloads for extended durations. Installations are supposed to be designed to avoid this. 2.5mmT&E is usually rated at a max of about
20A continuous. If this feeds *one single* 13A socket the current is limited to 13A by the fuse in the appliance plug. If this feeds *one double* 13A socket the current could theoretically be 2x13A=26A which exceeds the 20A rating of the cable. However it is usually assumed that a double socket will not be loaded to more than 20A. This is the situation for a spur from a ring and the reason why spurs are limited to one single or double socket. Caveats: (a) in kitchens etc washing machine and tumble dryer on the doubel socket may exceed the 20A rating of the spur cable; this is a design fault as a spur would be inadvisable in a kitchen or similar location. (b) if additional sockets are added to a spur or to a 2.5mm radial, the overload current of /n/ sockets x 13A rises above 20A and overload protection by means of the plug fuses provides insufficient protection. If a radial is wired in 2.5mm it should be protected by 20A circuit fuse, or 4mm for 32A.

However the 20A/2.5mm and 30A/4mm radials are "standard circuits"; it is quite possible the OP's is not a standard circuit and might still be compliant depending on the nature of the load.

Owain

Reply to
Owain

What Andrew is saying is that you need two types of protection, "fault" (i.e. short circuit caused by someone sticking a nail into the cable), and "overload" (i.e. when someone plugs in two 3kW fan heaters with a socket adaptor). With a typical ring circuit the MCB provides both of these types of protection at the origin of the circuit. However there are some cases where it is allowable to have the overload part of the protection at the usage end of the circuit when it can be shown that this protection ensures the cable can not be overloaded.

With fault protection you need the power to be cut quickly before the cable has a chance to melt or burst into flames as a result of the massive current that can flow (100's or 1000's of amps). You can calculate a "fault withstand time" (i.e. how long the cable will survive) using a thing called the adiabatic equation by feeding it details of the prospective short circuit current and the type of cable insulation material. What you find for short lengths of 2.5mm^2 T&E is that a 32A breaker will operate plenty fast enough with a short circuit on the end of the cable to ensure the cable is not destroyed.

Overload protection needs to ensure the cable is not slowly roasted as a result of excess load. So using the circuit you describe, (a radial with

2.5mm^2 cable) this would typically need to be protected by a 20A MCB since otherwise it would be easy to plug in (say) a total load of 40A spread over a few sockets. This would overload the cable but not open the 32A breaker that quickly (if at all).

With a spur (from a ring) or the particular setup described by the OP there is only one outlet and this is fused. Hence it is not possible to load the cable beyond what the 13A fuse on the outlet will carry for a sustained period.

it applies in both cases.

Reply to
John Rumm

Hmm,

Thanks for that John, but......

Surely, if it's a double socket (still permitted I beleive) then that could potentially be 2 x 13A if the spur is unfused.

Also, in the case the OP quoted, the main problem he has is he doesn't know what or how many outlets he has on the cable.

Reply to
cpvh

You are correct. There may be 26 amps passing through the spur cable at maximum load. I am not sure what people will find to plug in to create such a load but where there will there is a way. Fortunately on many installations the 2.5 T&E is conveniently rated at 27 amps.

Adam

Reply to
ARWadsworth

Strangely enough, a double socket is only required to be designed to take a total of (IIRC) 20 A. But yes, theoretically it could be loaded up to 26 A (or more - a BS 1362 13 A fuse will carry more than 13 A for quite some time).

This is just about fine on a 2.5 mm2 T&E radial provided the cable isn't buried in insulation etc etc, but definitely not recommended to run that kind of load for an extended period, even on a double socket on a ring main or 4 mm2 radial.

Reply to
Alistair Riddell

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