Omission of overload protection for fixed loads.

[Outspoken]

ok using your figures:




low and medium in parallel, 1/9.8 + 1/6.75 = 1/total, = 0.1 + 0.15 = 0.25, 1/0.25 = 4 ohms total.

230/4 = 57.5A , = circa 13kW

I think you should re read my post and I will edit mine as I forgot to divide 1 by the total....

 

[spark 68]

I think you should re read my post and I will edit mine as I forgot to divide 1 by the total....

That's what I said, you had forgotten to reciprocate the summation.

 

[Outspoken]

That's what I said, you had forgotten to reciprocate the summation.

Thanks, I have corrected it now if you look...I didn't spot my mistake as I was trying to put the post together, deal with English grammar issues for the 13 year old and stop the 3 and 4 year old fighting over toys and driving me mad...Mum is out and Dad got the baby sitting job, which I am not great at...tolerance is low for squabbles and unnecessarily shouting!! I did not recheck before posting...

Things like that I usually do in word first and double check it before posting....:90:

 

[Outspoken]

Are we forgetting that showers have a thermal cut out for any occasion when the elements overheat due to lack of water if the shower doesn't have a pressure switch or the breakdown and part shorting of the coils of the element....

They do, but if the Thermal gives way that could be the catalyst for the switch to fail due to overheating...

 

[spark 68]

The last shower I did was of the two element variety, and I was almost certain that on the higher power setting it just brought in the second element in parallel, as a boost so to speak, I could be wrong here, and I am not saying all showers are wired this way, I will have a look next time I install one.

 

[Outspoken]

The last shower I did was of the two element variety, and I was almost certain that on the higher power setting it just brought in the second element in parallel, as a boost so to speak, I could be wrong here, and I am not saying all showers are wired this way, I will have a look next time I install one.

To be honest I think this is how most showers work because it does remove the possibility of a fault inside the unit causing a serious overload, or it could be easier from a manufacturing point of view, but without physically taking each and every shower apart and checking the manufacturers technical information then the majority of sparks simply won't know and I believe it is incumbent upon us to assume that in the event of a switch failing for whatever reason it is possible to cause an overload by multiple coils being connected in parallel...unless we have clear evidence to the contrary from the manufacturers.

 

[Darkwood]

I think your trying to over think this whole situe', the design of showers will have to comply to there own BS regulations and if multi-element then they will have a fail safe system to ensure overloading cannot happen, i find it strange that your set up has 3 stand alone setting each running on its own for a given KW rating, id be more incline to believe that the elements work together so full power is achieved by all element running. Ie. 7.5kw for element 1 and element 2 and 3 are 1.5kw each as an example. Your example outspoken would be inherently dangerous if it was possible for all elements to come on together... the least of your worries is a hot cable if your user is in intensive care having skin grafts done.... i see you are trying hard to see issues with my theme of the thread but all this has been debated time and time again especially by the IET and yet the regulation still exists, we shouldn't be stripping down the working of a shower here as thats for the manufacturers to ensure for safe fail safe operation and/or when things do go wrong it wont present a danger to the user other than a sudden cold shower.

 

[Outspoken]

I think your trying to over think this whole situe', the design of showers will have to comply to there own BS regulations and if multi-element then they will have a fail safe system to ensure overloading cannot happen, i find it strange that your set up has 3 stand alone setting each running on its own for a given KW rating, id be more incline to believe that the elements work together so full power is achieved by all element running. Ie. 7.5kw for element 1 and element 2 and 3 are 1.5kw each as an example. Your example outspoken would be inherently dangerous if it was possible for all elements to come on together... the least of your worries is a hot cable if your user is in intensive care having skin grafts done.... i see you are trying hard to see issues with my theme of the thread but all this has been debated time and time again especially by the IET and yet the regulation still exists, we shouldn't be stripping down the working of a shower here as thats for the manufacturers to ensure for safe fail safe operation and/or when things do go wrong it wont present a danger to the user other than a sudden cold shower.

The regulation exists because there are uncommon situation when such an arrangement would apply, but I firmly do not believe a domestic shower is one of them.

You may think I am over thinking this, but if you do not think through every possible scenario then you will miss something that causes a problem. You can think that my shower is dangerous all you like, but it has a BS EN number attached to it thus is clearly is compliant. It has an overload attached (thermocouple type) and I understand and appreciate the chances of the worst case scenario happening are ultra slim so I do not see the danger your now saying exists, but the fact that the shower, in extreme situations, is capable of causing an overload is clear evidence that we cannot blindly apply regulations that suit our purposes because we feel our interpretation is correct. I know you have said you would not do it and feel that the reg was not written with domestic in mind, but then your thread and argument is justifying others doing it, and if they do not understand the dangers properly then you have the potential for disaster.

 

[Gavin A]

They do, but if the Thermal gives way that could be the catalyst for the switch to fail due to overheating...

for that to happen both the pressure sensor and thermal cut out would have to fail at the same time.

can't say that I can see how overload protection for the circuit would help in that situation either tbh.

 

[jamie-spark]

Is it just me , or is reg 433.3.1
(ii)For a conductor which, because of the characteristics of the load or the supply, is not likely to carry overload current, provided that the conductor is protected against fault current in accordance with the requirements of Section 434

Is this not utilised in domestic situations all over the country , an unfused spur off a ring final circuit Iz only needs to be equal to or greater than 20a

 

[Outspoken]

for that to happen both the pressure sensor and thermal cut out would have to fail at the same time.

can't say that I can see how overload protection for the circuit would help in that situation either tbh.

You are assuming every shower has a pressure sensor for a start, and secondly this will depend on how they are all connected up, many showers have electronic controls these days, if they go ---- up then it could be a case of none of the safety devices work...

 

[Outspoken]

Is this not utilised in domestic situations all over the country , an unfused spur off a ring final circuit Iz only needs to be equal to or greater than 20a

Thats why I have NEVER installed an unfused spur from a ring. I was taught as an apprentice that was the fastest way to an arse kicking and all spurs have always been fused. If you do not want to fuse it, you make it part of the ring.

 

[Gavin A]

You are assuming every shower has a pressure sensor for a start, and secondly this will depend on how they are all connected up, many showers have electronic controls these days, if they go ---- up then it could be a case of none of the safety devices work...

I really think you're barking up the wrong tree here.

standard shower protective devices are pressure sensor / flow sensor, over heat cut out, and pressure relief valve, or equivalent (eg an electronic control would presumably cut out if the electronic control failed) any shower without these (or at least 2 of the 3) would be a trading standards job IMO.

and shower elements will be nichrome, which have a temperature coefficient of resistance of 0.00017, so even if it's at 100degrees above it's starting temperature it will only increase the resistance by 0.17%. heating elements wouldn't be a lot of use if they were made of a material who's resistance increased that significantly as their temperature increased.

So a shower is very unlikely to operate for long while seriously over heating, and an overheating shower element won't draw any significant additional current anyway, so it's irrelevant to whether overload protection on the circuit would be required IMO.

 

[44ALLAN]

Before posting this I went back to my college/apprentice days and took the thing apart, completely.................

Gawd...........I still do that now sad or what

 

[Outspoken]

I really think you're barking up the wrong tree here.

standard shower protective devices are pressure sensor / flow sensor, over heat cut out, and pressure relief valve, or equivalent (eg an electronic control would presumably cut out if the electronic control failed) any shower without these (or at least 2 of the 3) would be a trading standards job IMO.

and shower elements will be nichrome, which have a temperature coefficient of resistance of 0.00017, so even if it's at 100degrees above it's starting temperature it will only increase the resistance by 0.17%. heating elements wouldn't be a lot of use if they were made of a material who's resistance increased that significantly as their temperature increased.

So a shower is very unlikely to operate for long while seriously over heating, and an overheating shower element won't draw any significant additional current anyway, so it's irrelevant to whether overload protection on the circuit would be required IMO.

I am not sure that is accurate. Seems I will be investigating my shower again before I put it back together...

 

[Gavin A]

I am not sure that is accurate. Seems I will be investigating my shower again before I put it back together...

circuit diagrams are your friend here.

there are many of them available via google, and I've just checked several to be sure. Regardless of that though, the point you were making is wrong as even an overheating element won't add more than 0.2% or so to the load of the circuit.

 

[telectrix]

slightly off topic. but this might be of interest. came across a downflow heater 2kW. the L termination in the FCU had burnt out, taking about 1" of the L of the flex with it. now the flex on the heater ( came with it attached by manufacturer ) is 0.75mm and manufacturers instructions specify a 13A fuse. are they using 433.3 to get away with such a small csa? and who is liable if the 0.75 melts before the fuse blows?

 

[Darkwood]

The heater manufacturers will have to comply to their own BS standards, the flex will be compatible for the load of the heater so they won't be responsible, clearly the issue here is a connection problem or a failed FCU by matter of interest i bet the multi-stranded cable termination into the fcu wasn't appropriately crimped for termination so if so then the installers fault, if it was down to failure of the FCU then the FCU manufacturers fault, proving it and laying blame isn't usually worth the hassle on a simple repair. For your query regarding using 433.3 they are not subject to the BS7671 but thats not to say they dont have this regulation in their own BS standards as clearly they have omitted overload protection of the flex itself. Having said this this wasn't caused by overload but thermal conductivity from a poor contact or connection in the FCU which regardless of any overload protection this would have created the same effect even with a fuse rated spot on for the load.

 

[diabalo]

Great thread, been following this on the IET forum too. Even as a diyer I follow the technical arguments and have given a couple of examples on there (where I am known as rovermaestro) of circuits in my current house where the ccc is less than the copd, 8.5kW shower on 6mm in trunking on a wall with 40A rcbo and a 4mm radial 2 seperate sockets (washer and tumble drier) 22A load, ccc 30A (trunking on a wall) copd 32A. I had the CU changed last year after an EICR and although noted these circuits were reconnected after discussion. (I didn't install them). I think this was eminently sensible and I am absolutely happy with this arrangement in practice. Seems to me that designers have quite a lot of scope within the regulations to make these judgements within the standard. As a householder I am conscious of the risk of future additions or replacements but whoever does that is responsible for checking the circuit characteristics before taking action. I think decent paperwork is vital. As others have stated elsewhere every installation should have a proper descriptive document. I would regard it as an essential user manual.

 

[KAS1]

I think that has always been a flaw when it comes to domestic works, the lack of a design document to accompany each installation should of been compulsory many years ago, not just a record of test results but a full spec as in commercial / industrial installations. After all it wouldn't of taken much design work for the average domestic IMO