Part P testing for non standard wiring

On Tue, 01 May 2007 22:50:26 +0100, "Dave Plowman (News)" mused:

Dodgy neutral connection, burnt neutral. Dodgy live connection, burnt live.

I've seen loads of burnt stuff, all colours, voltages, phases and otherwise.

I expect they take extra care to tighten up the "live" because that one could get hot.

Owain

I've heard that said before, and have also found loose neutrals on occasion. Perhaps there's a subconscious tendency to take more care in tightening phase terminals because they are "~~~live~~~", and earths because they're for safety, whereas the poor old neutral goes to the bottom of the heap?

Not at all. You want me to do it?

I'm afraid I have to disagree again. Logically there is no difference in the above two situations, but technically I believe there is. You could rely on BS7671 as a defence in court for the unprotected spurs, but not the unprotected radials.

Did I mention I don't like ring circuits?

Regards

T

They are an excellent solution (in the whole as implemented in the UK) for domestic use. You just need to see the bodges in other countries to realise why.

And should the connection fail, your entire circuit would be unprotected from overcurrent, and you wou would be none the wiser. With a radial you would know. If the "alternative path" works so well at protecting the circuit, how on earth would you spot a high impedence connection? A connection is either high impedence or not, irrespective of layout. Localised failure may result irrespective of circuit layout. The result is safer if the circuit is radial.

Either the CPC is high impedence or not. If it is high impedence then it is so whatever the circuit layout.

This is true. However the remaining earth on the ring may or may not provide the required protection.

And the risk of your house burning down. Accelerated cable fatigue and its associated risks are perfectly possible on a correctly installed ring. Try connecting 2x3kW devices on a spur!

"Outside" means not part of the fixed installation. A circuit is defined as "An assembly of electrical equipment supplied from the same origin, and protected against overcurrent by the same protective device(s)". The two circuits you suggest above are non-compliant.

If I wanted my 32A radial circuit extending, and I got a bill for some

2.5mm cable and a new 20A breaker, I think I would have to be physically restrained!

Not only is that sort of thing shoddy and pointless, the sort of electrician who would do this would doubtless leave no documentation. At some future date the next electrician could be forgiven for relacing the breaker with a 32A, then the 2.5mm cable would be endangered - just like a spur!

The _only_ occasion derating a cable, to below the breaker capacity is permitted, without further protection is a spur. A spur is defined as a branch from a ring final circuit. There are restrictions on the number of these, and what can be attached to them.

Your suggestion does not comply with BS7671 (for obvious reasons).

Please point me to anywhere in the regs where protection against overcurrent by a CPD at the far end of a cable is permitted.

So you found two non-compliant circuits. I guess in your job this happens all the time.

How about fitting two 20A mcbs? How about checking if 20A is required (which I doubt), and if not, fitting one 20A mcb. How about "spurring" the pump off the ring at the consumer unit (the bitter irony)?

I trust the pump was on an RCD protected circuit?

So we are agreed than that a 32A (4mm) radial would be much better in this case.

Regards

T.

Sorry, can't see that. Since we are talking about a general purpose power circuit here, the whole circuit still has overcurrent protection provided by the MCB, as does each individual appliance connection via its fuse. A radial in the same situation would be likewise protected, however the probability is that more circuit current will pass though your high impedance connection since there is no alternate low impedance path.

Why would you be any more likely to know you had a connection problem on a radial? You only need fractions of an ohm to result in significant heating at a socket, and that will not cause sufficient voltage drop on the end of your radial to be noticed. The only way you will detect a fault situation in either case is with routine maintenance or by testing. In domestic situations it is safe to say the former hardly if ever happens. Testing is only likely when changes are made to the circuit.

Chances are you would not. Same as with a radial.

Why, because you will get more localised heating?

No, on a radial a high impedance joint will result in a high loop impedance for all connection points downstream of the fault location. On a ring this is not the case. (one of the reasons that the CPC is connected as a ring even on radial circuits, where high integrity earthing is required)

Shall we see?

Assume a TN-S install with Ze of 0.8 ohms. Take a circuit length of say

20m, and to get worst case we can ignore the parallel path. This lets us use the R1 + R2 values from table 9A of the OSG. on 2.5mm^2 cable we have a round trip resistance of 19 mOhm/m, or 0.38 ohms. Add to Ze to get Zs of 1.18 ohms. The prospective fault current at 230V would therefore be 195A. On a type B 32A MCB this places it into the instantaneous trip part of its curve (BS7671 fig 3.4). So lets take that as 0.1 seconds to clear the fault current. This gives us:

s = sqrt( 195^2 . 0.1 ) / 115 = 0.5mm required csa of the CPD. 2.5mm^2 T&E typically has a 1.5mm^2 CPD so that should be adequate even of one path were completely open circuit.

6000 / 230 = 26A, or a bit less than the rating of the cable at 28A then. That should not cause any particular problem for the cable unless there is another reason to de-rate it (in which case use a bigger cable).

The thing to bear in mind is that the difference in thermal capacity between a 2.5mm^2 cable and a 4mm^2 one is not that huge. Dissipate 300W in a poor joint and both are going to get very hot, and both are clad with the same 70 degree PVC insulation.

So the circumstance I describe with a FCU providing the overcurrent protection at the appliance connection point, this would be "inside" by your definition.

[434-01-01] requires that the circuit be protected against fault current at the origin. However it permits that "the nominal current of such protective device may be greater that the current-carrying capacity of the conductor being protected". A 32A MCB will provide adequate fault current protection to a single 2.5mm^2 T&E (spurs using it would otherwise not be allowed). [432-03] allows for "protection against overload current only". It allows the overcurrent protective device to also have a lower breaking capacity that the prospective fault current of the circuit in circumstances where fault protection is provided by a separate device. [473-01-02] specifically allows for the overcurrent protective device to be placed anywhere along the run of the conductor (providing there are no upstream branches feeding other outlets etc).

We seem to be talking at cross purposes. I was simply highlighting that a blanket "rule" of not being able to reduce cable cross section without additional protection is not actually valid on a ring or a radial.

For example, say you needed to replace a segment of cable on a 4mm^2 radial (protected by a 32A MCB). It would be acceptable to replace the section with 6mm^2, and keep the 32A MCB.

There would be no need to provide additional overcurrent protection at a point where the circuit reduces from 6mm^2 to 4mm^2 since the MCB is already correctly sized to protect the 4mm^2 cable.

The example you give (reducing the effective capacity of the whole circuit to cope with a section of smaller than originally designed cable), may not be desirable, however if for some reason you were forced into this course of action, it would be the correct way to do it.

I don't expect many electricians would do this, unless forced by circumstance (for example having to use an existing spare cable run in a circumstance where it is no longer possible to get a new cable in place)

Indeed, but why would you carry out said modification unless forced? If forced one would record the reason on the test results and also ideally a notice on the CU.

Could you explain this?

A spur, if correctly implemented is protected from both fault current and overcurrent - just not by the same device.

The cable current capacity is permitted to be below the nominal current of the MCB, if the MCB is only providing fault current protection, and overload protection is provided elsewhere. This is the case with a spur.

There are no hard restrictions that I am aware of, although there are guidelines as to the proportion of spurs to "normal" outlets on a ring circuit. Obviously when designing a new circuit it makes sense to use a few spurs as reasonably possible, since it allows for more flexibility when implementing extensions and modifications later.

I think you will find it does, however if you can point out where is does not, feel free.

References given above. However for a simple example think of an unfused spur.

Compliant, but unusual.

(in this particular example, I moved them to dedicated 16A MCB protected radials anyway since there was space in the CU, and there were other design faults that had been grafted on at a later date)

Curious assumption...

Why do you suppose in "my job" this "happens all the time"?

one could do, I opted for 16A.

It wasn't, hence why I selected 16A

Why, the pump was nowhere near the consumer unit? (it was in the airing cupboard - the only circuit present there being for the immersion heater)

As it happens, the whole house was protected by a single RCD, although there was no particular reason this circuit required additional protection from an RCD.

Slightly better in this restricted circumstance, however a little more though on circuit layout would solve the problem.

On 2 May 2007 02:04:23 -0700, snipped-for-privacy@gmail.com mused:

If it didn't comply to BS7671 then it wouldn't stand up in court IMO, whether it was technically right or not, in the main.