Myths

[JohnW2]

Yes, sorry again. I forgot about the derating.

Fair enough.

The more I read the regulation, the dafter it gets.

The aspect we are discussing certainly does seem daft, but other aspects are, at the least, a little odd, not to mention that it's all badly worded. Particularly odd is the T+E/MICC difference, for which I don't think anyone has managed to produce any half-credible 'explanation'.

Kind Regards, John

 

[JohnW2]

I do not.

That's a bit ambiguous, given the negatives in the statement to which you are responding - could you possibly clarify? Are you saying that, in the same way that you seemingly do not think that the "20A minimum CCC" applies to spurs, nor do you believe that the 2.5mm² minimum (other than for MICC) applies to spurs?

There is no reason it should apply to the ring using 32A MCBs as opposed to 30A BS3036s for wich the regulation is still written.

It does seem very odd that [no matter which cable(s) they are talking about] they specify the same 'minimum CCC' for both 32A MCBs and 30A BS3036 fuses.

Just one fairly wild thought - although, again, it would be very bad if they had not been clear about this, is it conceivably possible that when they talk about the "CCC" of the cable, they are thinking of the CCC as corrected, if necessary, by 433.1.202 in the case of a BS3036? - which would (after correction) be around 20A for 2.5mm² T+E ??

Yes, I would as long as the derating and fault conditions are met.

That would certainly be logical, since it would then be the same as when there were no ring. However, I imagine that, even if it were only supplying one single socket, a good few eyebrows (particularly of EICR inspectors!) might rise if they found a 1.0mm² spur from a ring final

Kind Regards, John

 

[JohnW2]

The ,quote of 433.2.2> ......... I am told does not include unfused spurs, I have not yet worked out why this does not limit the length of an unfused spur, ....

In order to 'work out why', you only have to read the reg which you quoted

It says that downstream overcurrent protection is acceptable if the situation ".... fulfils at least one of the following conditions:", the first of which conditions is:
(i) It is protected against fault current in accordance with the requirements stated in Section 434
So, provided that there is adequate fault protection (which there almost always will be), the second condition (limiting length to 3m) does not have to be satisfied.

"Simples"

Kind Regards, John

 

[Murdochcat]

There is no consensus about such things, but the 'rating' of the socket does not limit the possible overload current, which (until an OPD operates) is determined solely by the load.

BS1362 fuses being as they are, if two loads were plugged into a double socket with plugs which both had 13A fuses, a total of around 44A could theoretically flow for an indefinite period of time!

Kind Regards, John

well possibly BUT my money is on the fuses in the plugs failing long before the cable

this is yet another example of poorly worded, and never improved regs.

Why should anybody surprised?

 

[ebee]

Fair enough. There are some stations in which the regs allow omission of overload protection, primarily when the nature of the load is such that it is 'unlikely to result in an overload current' (usually purely resistive loads) - but, as you will often see here, it is nearly always a contentious issue, with some people always willing to think up ways in which almost any load can (very rarely) result in overload current.

Interestingly, although you mentioned it as 'one of the 3 protections', BS7671 really only talks about overload and fault protection. I don't think there is any situation in which BS7671 permits the omission of both overload and fault protection and if either (or both) of those is present, that will inevitably also provide 'short-circuit protection' - so that doesn't really need to be considered as a separate 'protection'.

I'm nor sure what you mean by 'fixed' but, as above, there are limited situations in which overload protection may be omitted, and most of those are to some degree contentious. If the load is not purely resistive, and particularly if it involves significant motors, overload protection is likely to be required.

Kind Regards, John

A fixed load would be for example a resistive heater (shower/immersion heater for example) that load is fixed and if you limit the number connected to a circuit (in the examples one) then you have limited the load by design and it can not overload therefore overload protection is not required.

Therefore your OPD is only required for a max of two conditions - short circuit and earth fault - you may or may not address both/either by other means or you might still use the OPD for these features but also suppliment either or both of those conditions by protection by other means.

RCDs are obvious examples - we usually protect against all three conditions by the OPD (MCB/Fuse usually) but in a TN system we add an RCD to suppliment earth fault protection. In a TT system we usually have not much choice but to use an RCD for the sole means of earth fault protection.

When doing so I prefer to have two RCDs in series although one of them might be 100mA Time delayed (Not actually rated as personal protection because it would not be over helpfull to protect you against something you are already in contact with but if the fault develops before you touch it then it might clear that fault beforehand therefore sometimes might be helpfull)

 

[ericmark]

In order to 'work out why', you only have to read the reg which you quoted

It says that downstream overcurrent protection is acceptable if the situation ".... fulfils at least one of the following conditions:", the first of which conditions is:
(i) It is protected against fault current in accordance with the requirements stated in Section 434
So, provided that there is adequate fault protection (which there almost always will be), the second condition (limiting length to 3m) does not have to be satisfied.

"Simples"

Kind Regards, John

434.2.1 The regulations in Regulation 434.2 shall not be applied to installations situated in locations
presenting a fire risk or risk of explosion and or where the requirements for special installations and locations specify
different conditions. Amended July 2008

Except where Regulation 434.2.2 or 434.3 applies, a device for protection against fault current may be installed
other than as specified in Regulation 434.2. under the following conditions:

The part of the conductor between the point of reduction of cross-sectional area or other change and the position of
the protective device shall:
(i) not exceed 3 m in length, and
(ii) be installed in such a manner as to reduce the risk of fault to a minimum, and
NOTE: This condition may be obtained. for example, by reinforcing the protection of the wiring against external influences
(iii) be installed in such a manner as to reduce to a minimum the risk of fire or danger to persons.

so still limited to 3 meter!

 

[JohnW2]

well possibly BUT my money is on the fuses in the plugs failing long before the cable

I don't doubt it. Amidst all the discussions/arguments we see about slight (albeit strictly true) deviations from the regs, I hink we can lose sight of the fact that there are undoubtedly some massive 'safety margins' built into figures we work with )particularly CCC figures). It also depends upon what you mean by a cable 'failing' - I think the fusing ('melting') current of, say, a 1.5mm copper conductor is around 130A, and a 13A BS1362 fuse would certainly blow long before that happened.

this is yet another example of poorly worded, and never improved regs.

Indeed.

Why should anybody surprised?

They shouldn't ...... who is 'surprised', I wonder?

 

[JohnW2]

so still limited to 3 meter!

Not if, as will nearly always be the case, fault protection is provided by an upstream device.

Kind Regards, John

 

[JohnW2]

A fixed load would be for example a resistive heater (shower/immersion heater for example) that load is fixed and if you limit the number connected to a circuit (in the examples one) then you have limited the load by design and it can not overload therefore overload protection is not required.

Those are the two examples I gave, and the most important point is that they are resistive loads, of fixed resistance during normal operation, and I am personally happy to believe that it is (very) 'unlikley' that they could result in overload currents.

However, you must surely be aware of the fact that, whenever that concept is mentioned, people like SUNRAY and bernard scrape thee barrel and postulate scenarios (I would think incredibly improbable, since certainly not 'likely') in which an overload current (not a short-circuit or fault to earth) can theoretically arise in the elements of things like immersions and ovens - primarily situations in which three is a 'looped' healing conductor, two parts of which come into contact (thereby 'short-circuiting' a portion of the resistance/impedance).

When doing so I prefer to have two RCDs in series although one of them might be 100mA Time delayed (Not actually rated as personal protection because it would not be over helpfull to protect you against something you are already in contact with but if the fault develops before you touch it then it might clear that fault beforehand therefore sometimes might be helpfull)

As I often say, I will never knock caution or 'belt and braces', and redundancy of RCDs is a concept with which I can sympathise, given the allegedly significant 'failure rate' of RCDs. In my installation, I have just as you describe, but necessarily because ut is TD. Although all final circuits have 30MA RCD/RCBO protection, I also have to have 100 mA TD RCDs 'up-front', providing fault protection to the lengthy distribution circuits.

Kind Regards, John

 

[EFLImpudence]

so still limited to 3 meter!

No, because:

"434.2.1 The regulations in Regulation 434.2 shall not be applied to installations situated in locations
presenting a fire risk or risk of explosion and or where the requirements for special installations and locations specify
different conditions. Amended July 2008.
Except where Regulation 434.2.2 or 434.3 applies, a device for protection against fault current may be installed
other than as specified in Regulation 434.2. under the following conditions:
The part of the conductor between the point of reduction of cross-sectional area or other change and the position of
the protective device shall:
(i) not exceed 3 m in length, and
(ii) be installed in such a manner as to reduce the risk of fault to a minimum, and
NOTE: This condition may be obtained. for example, by reinforcing the protection of the wiring against external influences
(iii) be installed in such a manner as to reduce to a minimum the risk of fire or danger to persons."


"434.2.2 The device protecting a conductor may be installed on the supply side of the point where a change
occurs (in cross-sectional area, method of installation, type of cable or conductor, or in environmental conditions)
provided that it possesses an operating characteristic such that it protects the wiring situated on the load side against
fault current, in accordance with Regulation 434.5.2."

I.e. the circuit protective device in the CU.

 

[JohnW2]

..... I.e. the circuit protective device in the CU.

Quite. As I said, adequate fault protection will virtually always bee provided upstream, by a device in the CU/whatever.

Kind Regards, John

 

[EFLImpudence]

Yes, but I am trying to persuade Eric.

 

[JohnW2]

Yes, but I am trying to persuade Eric.

I realise that. We've both tried - so maybe, between us, we'll achieve something

I have to say that I wonder how often it would be necessary to invoke 434.2.1 (to allow downstream fault protection) since, as I've said, I would think that adequate fault protection would almost always be afforded upstream by a device at the origin of the circuit ... or am I missing something?

Kind Regards, John

 

[SUNRAY]

Those are the two examples I gave, and the most important point is that they are resistive loads, of fixed resistance during normal operation, and I am personally happy to believe that it is (very) 'unlikley' that they could result in overload currents.

However, you must surely be aware of the fact that, whenever that concept is mentioned, people like SUNRAY and bernard scrape thee barrel and postulate scenarios (I would think incredibly improbable, since certainly not 'likely') in which an overload current (not a short-circuit or fault to earth) can theoretically arise in the elements of things like immersions and ovens - primarily situations in which three is a 'looped' healing conductor, two parts of which come into contact (thereby 'short-circuiting' a portion of the resistance/impedance).

As I often say, I will never knock caution or 'belt and braces', and redundancy of RCDs is a concept with which I can sympathise, given the allegedly significant 'failure rate' of RCDs. In my installation, I have just as you describe, but necessarily because ut is TD. Although all final circuits have 30MA RCD/RCBO protection, I also have to have 100 mA TD RCDs 'up-front', providing fault protection to the lengthy distribution circuits.

Kind Regards, John

I've been watching this progression, brilliant to see it running so well. The only reason I write about the emboldened subject is personal experience. I've encountered the issue with trace heating cable (albeit usually as a result of damage) and heater batteries in air ducts for example. Certainly one oven element but the replacement was a different design, sadly the reason the fuse blew/mcb tripped was not due to the element overload directly, it was due to the damage caused to the overloaded wiring within the oven whose only protection was the large CU device. Furthermore it was not covered under warranty as MI's had not been followed.

 

[JohnW2]

I've been watching this progression, brilliant to see it running so well. The only reason I write about the emboldened subject is personal experience.

Yes, as we've often discussed, I understand that.

As a matter of interest, are there any situations in which you feel it acceptable to omit overload protection? If not, is it your feeling that 433.3.1(II) should not exist?

Kind Regards, John