Omission of overload protection for fixed loads.

[Darkwood]

Been reading various threads with regards to having an mcb that exceeds the cable rating thus only providing short circuit protection, what surprises me is the negative responses that are posted if it is ever suggested.

Now lets try put a translation on the regulation specifically 433-3-1 (ii)

A device for protection against overload need not be provided:

(ii)- for a conductor which because of the characteristics of the load or the supply , is not likely to carry overload current, provided the conductor is protected against fault current in accordance with the requirements of section 434.

This particular regulation is can be applied when the load is fixed or by the nature of the load and its control system an overload is unlikely to occur as in a motor circuit where an overload device will be monitoring the motor in its control system. It can also be used for say a shower circuit or water heater etc however having said this it is however usual to provide overload protection unless it is impracticable to do so.

What im getting at here is you really shouldn't be applying this regulation to your domestic shower if your are wiring it but it can sometimes be utilised if say you fit a larger shower within the ccc of the cable but by the nature of mcb ratings you find your mcb slightly over the ccc rating of the cable.... but all other calcs must comply as normal. I want to stress it shouldn't be applied routinely in domestic install but is permitted but as a rule don't use it in your designs. Where this regulation comes into more common use as i do often have to imply it is supplying motors and machinery and by the nature of inrush a mcb (even d rated) would nuisance trip because it is lower than the ccc of the cable it protects... fitting a higher rated mcb can omit nuisance tripping and overload isn't an issue as the motor has its own overload protection.

So please can we stop throwing this regulation at members when discussing showers etc as this becomes misleading to the OP who might not be as clued up on the nature of this regulation.... yes its allowed but it shouldn't be practiced in the domestic realm.

 

[somersetsparks]

I regularly apply this to 8.5kw showers wired in 6mm T+E on a 40A MCB on EICR's, however only a fool would actually install it new in 6mm.

 

[KAS1]

Very well put

 

[Darkwood]

As i said sometimes its impractical to fully rip out a 6mm and install a 10mm just because your upgrading the shower by half a KW and stay within the ccc of the cable and zs etc complies so yes can be utilised but if you are calculating the cable size to wire it new then you should be providing overload protection of the circuit cable and not using this regulation as part of your design, you should also be considering future issues if you do take this route i.e. if it goes through a loft which at present has no insulation then you should assume and calculate for the future addition of insulation as this could create a fire hazard as the cable has no overload protection. Hence i stipulate it should only be used in domestic with great consideration.

 

[Marvo]

I would say in a domestic situation there's a good possibility an additional circuit may be spurred of an existing circuit at any time in the future therefore no domestic circuits would meet the criteria of being unlikely to carry an overload current.

 

[somersetsparks]

I would say in a domestic situation there's a good possibility an additional circuit may be spurred of an existing circuit at any time in the future therefore no domestic circuits would meet the criteria of being unlikely to carry an overload current.

Fair point, I see no need in a domestic installation situation to ever use 433-3-1. The standard circuits defined in the onsite guide are all that should be used in a new build/re-wire, However for an EICR its a different matter.

 

[Outspoken]

So please can we stop throwing this regulation at members when discussing showers etc as this becomes misleading to the OP who might not be as clued up on the nature of this regulation.... yes its allowed but it shouldn't be practiced in the domestic realm.

With respect, that is only YOUR opinion and I personally consider it incorrect as such if I see people stating things I believe to be dangerous, non-compliant or simply incorrect I will point this out and I'll be damned if I will be silenced by someone who simply also feels they are right. The belief that a shower cannot cause an overload is electrically incorrect and as such the regulations require overload protection for the cable, it is not for you or I to decide the Regs have it wrong.

 

[kingeri]

Completely agree with darkwood. A shower is a fixed load and hence we know it can't draw more than it states on the tin. The cable supplying it obviously needs to be beefy enough to handle this load, but strictly speaking the protective device needs only to provide fault protection. Would I install a shower in this way? No. If I did, would it be compliant work? As far as I can see, yes.

 

[Darkwood]

Wasn't a personal vendetta 'outspoken' i actually forgot who wrote the recent replies but many have stated it, mine isn't a personal opinion itsfrom guidance notes printed in the IET electrical design guide and i re-iterate that they express you should be providing overload protection unless it is impractical as in motor circuits. I am not deciding the regs im quoting from the guys who wrote it. They do say fixed loads like showers and water heaters can be done this way but express that design should be with overload protection of the cable 433.2...i assume that you need to justify otherwise.

Their examples of calculations relate all to motor circuits which gives the impression this regulation isn't really about the domestic install but thats open to debate.

 

[Dubbo]

Doesn't this happen all the time when your spur of a 32 amp ring with 2.5mm rated at 27 amps

 

[Outspoken]

DW, I did not take it "personally", I understand the context you meant your comments in, but my reply was personal because I can only speak for myself as I have been so vociferous in my earlier replies in earlier threads on this subject, and will continue to be.

A lot of people have stated the following (Teacher head on now)

"A shower is a fixed load and therefore cannot draw more from the supply that it is rated at"

If you are an electrician and you make such a flagrantly ridiculous comment then shame on you because you are wrong. You are wrong because to make your statement without overload protection in place in rewriting the laws of physics and if you really think about it you will understand why you are wrong, because if you were correct then nearly every circuit in a domestic premises, or actually most circuits in most premises would not need overload protection because by their nature they are effectively fixed loads. Clearly circuits containing sockets of one type or another, or any circuit where the load attached to it can be variable would not be included in this, but the overwhelming majority of circuits would.

Please gentlemen, think about this fully and clearly, consider how a shower works and consider what possible external or internal influences could alter the load drawn by a fixed appliance of known electrical resistance.

If someone can honestly answer this and convince me that you are correct that nothing can cause this to alter the load drawn from the circuit then I shall concede, however I know the bedrock of my argument is as solid as a planet made of pure diamond. (Just in case you doubt an entire world can be made of what is effectively a C[SUP]8[/SUP] (8 Carbon atoms) tetrahedral crystal structure (diamond) then please READ)

 

[Darkwood]

Im quite fluent in physics thank you and study advanced quantum theory of which i have had a deep interest in for 25yrs.. and my comments are sourced from print in the IET Electrical Installation Design Guide covering the BS7671 and i quote 'BS7671 allows the use for any fixed loads for example water heaters; however it is usual to provide overload unless it is impracticable as long as your circuit complies to the equation It >or= Ib/Ca.Cg.Ci

 

[Outspoken]

Im quite fluent in physics thank you and study advanced quantum theory of which i have had a deep interest in for 25yrs.. and my comments are sourced from print in the IET Electrical Installation Design Guide covering the BS7671 and i quote 'BS7671 allows the use for any fixed loads for example water heaters; however it is usual to provide overload unless it is impracticable as long as your circuit complies to the equation It >or= Ib/Ca.Cg.Ci

Well then please explain why you believe your correct that no overload protection is required because the load drawn can never exceed that stated on the "tin"

 

[Darkwood]

I never stated it cannot exceed that stated on the tin... others may have.... im only quoting the designers of the regs.

 

[Outspoken]

I never stated it cannot exceed that stated on the tin... others may have.... im only quoting the designers of the regs.

DW, I certainly do not want to get into an argument over this, I hope we are both capable of debating this maturely and professionally, so lets both agree not to escalate the language we use.

You would appear to have backtracked in a couple of your posts. In your first post you are stating that the Regs clearly give a "green light" to the omission of overload protection for shower circuits because of the comments in 433.2.1, later you state your opinion is based on a guidance note, strange because I have checked mine and cannot find such a reference, and then you are stating you are not saying at all, but again quoting the "designers of the Regs".

This is not about the Regs, this is about simple electrical understanding and common sense. The Busbar in a DB or single phase consumer unit has a load carrying capacity that far an away exceeds that of the cables we install in any premises, because if they did not we would all be in trouble. Thus the connection of the circuit at the Bus Bar involves a change in the current carrying capacity of the circuit, and for compliance with the regulations we must install overload protection.

Now this point can be argued I agree, but there is a more fundermental issue at hand here, namely the voltage of the supply.

Yes we all know the national average voltage is nearer 237.6V and not the stated 230V we all do our calculations at, but within the UK there is variation, I have personally measured voltages on the mains as low as a static 220V and as high as 253V, and this is within the normal range the Regulations (not IET) allows. However under fault conditions it is possible for external effects to impact your supply, and I have personally seen faults on the local grid that mean the homes are only getting 160-200V, in fact a fault with the concentric cable feeding the average home today can develope a fault that can reduce the delivered voltage to 170V.

I appreciate we are talking about influences outside our control, but does this mean we ignore them because they are rare? I think not, or we would simply wire all circuits straight into a 100A BS88-3 fuse because faults on the individual circuits are rare...

Removed incorrect information.. (Must not post tables and think of things after a bottle of white!)


Of course there are internal issues with showers that can cause serious faults that result in overload. The shower water temperature is controlled by a thermocouple, if this fails, 9 times out of 10 the shower will stop working, which is a good thing from a safety standpoint, however I have known them fail closed so they allow the element to continue to receive a voltage and thus stay "hot". If the water is flowing this may not be an issue as the heat is carried away, but if the shower is left on with a reduced water flow it is quite possible for the heat to build up around the element changing it's electrical properties. This is worse in hard water areas were the element may have a covering of limescale that acts like a thermal blanket. The change in resistance caused by temperature may be subtle, but it could be enough to overload the circuit if it is "on the limit" and where you have only fault protection provided by an RCD it is possible for the cable to be damaged as a result. Such faults may not cause any long term damage to the shower unit, but repeated incidents could cause increasing damage to the supply cable. Eventually the RCD would trip, whoop-di-doo, however in the meantime another fault has gone unnoticed that has allowed this final fault to occur.

Prevention is better, this is why I argue for all showers to be properly protected by the use of RCBO's or perhaps a circuit breaker that is backed by an RCD main switch on the DB/CU. Anything less is not justifiable works in my humble opinion.

 

[electric will]

Quote,
Now look at this, we all know as the voltage changes on a fixed resistance the current drawn changes, and in the case of voltage drops the increase in current drawn

are you sure?

 

[Outspoken]

..............

Thanks Will, please see post #25 below :banghead:

 

[electric will]

On a resistive load if the voltage drops so will the current and also the output.

 

[spark 68]

That table is flawed Outspoken, that only holds true if the kW output stays the same.

In reality, assuming the resistance stays the same, if the voltage drops then so will the current, and consequently the kW output as P = V X I

I agree with Will.

Ps. I see you have edited your post.

 

[stuartcourt]

saw that edit as well, yet no mention of it under quote how