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...

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

There is a 32A breaker marked as Spare with a single 2.5mm wire inserted (So
it isn't spare then I assume!)

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

Now, one part of me says this is wrong, it should be a 20A MCB as it must be
a radial,

Correct.

but if this were a ring circuit, then I could spur off of it, so
the spur would be protected with a 32A breaker.

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.

The installation has passed safety inspections, should it have?

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


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

"David Hansen" wrote in message
...
On Tue, 21 Feb 2006 16:17:04 -0000 someone who may be "Sparks"
wrote this:-

There is a 32A breaker marked as Spare with a single 2.5mm wire inserted
(So
it isn't spare then I assume!)

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

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!

Now, one part of me says this is wrong, it should be a 20A MCB as it must
be
a radial,

Correct.

but if this were a ring circuit, then I could spur off of it, so
the spur would be protected with a 32A breaker.

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.

The installation has passed safety inspections, should it have?

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

I will find out what was tested then!

Thanks!

Sparks...

"David Hansen" wrote in message
...
On Tue, 21 Feb 2006 16:17:04 -0000 someone who may be "Sparks"
wrote this:-

There is a 32A breaker marked as Spare with a single 2.5mm wire inserted
(So
it isn't spare then I assume!)

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

Now, one part of me says this is wrong, it should be a 20A MCB as it must
be
a radial,

Correct.

but if this were a ring circuit, then I could spur off of it, so
the spur would be protected with a 32A breaker.

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.


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
current that can be drawn down the spur is limited. However, the
30/32A protective device doesn't protect the spur cable properly.

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

Adam

Sparks wrote:


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!


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

In article ,
David Hansen writes:
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

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.

moons ago, because at one time spurs could feed two single sockets

14th edition regs.

--
Andrew Gabriel

Andrew Gabriel wrote:
In article ,
David Hansen writes:
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

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.


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.

wrote:

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.



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?

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.


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

it applies in both cases.



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

John Rumm wrote:
wrote:

snip

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.


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.

wrote in message
oups.com...

John Rumm wrote:
wrote:

snip

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.


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.

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

On Wed, 22 Feb 2006, wrote:

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.

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

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.

--
Alistair Riddell - BOFH