I must need re education , then please,

[5544]

I have been inspecting and testing for some years and have until now believed that I have been doing it correctly. I spent some time at a property yesterday for a landlord doing a condition report for a rental property.
there were many many issues with several clearly being in my opinion C2. one point really confused me, (well not until today),
There was a standard electric hob at 6Kw , and a separate double oven at 4.4kw. Both were wired into their own 13amp SFU. the oven actually had 0.75mm flex going to a piece of 6mm T+E which then went to the 13amp fuse.
The oven sfu actually went back to a 6mm t+e 32amp supply to the consumer unit.
The hob was connected to the ring main.

I check the manufacturers instructions and they both stated a 32amp supply hard wired was needed.

I advised the Landlady who said the oven had been installed by e reputable supplier and had the safety certificate to prove it. She had also spoken to another engineer who had said that I was wrong to pick it up as the safety was the 13amp fuse, which would have blown.

I have issues with this, well all of it actually, but honestly, where does this situation stand with the condition report. In my opinion it is at least a C3 but possibly a C2.

I am not looking to get slagged off but just seeking some direction.

kindest regards

 

[telectrix]

C2 in my book. under rated fuses; undersized cable; 6kW hob on RFC; ignoring manufacturers instructions.

 

[SparkyChick]

You are the inspector... you need to be happy with the classification you put on it. Personally from what you've described... I'd C2 the two issues.

 

[Lucien Nunes]

The 0.75mm² cable is not protected against overload by the 13A fuse nor by the appliance being a fixed load of 6A or less. The fuse is not correctly selected because it is likely to experience long duration low overload (prior to the moment someone turns everything on and pops it.) Both of these discrepancies carry a risk of overheating and were never compliant. hence C2 for me.

 

[Rockingit]

Interesting. Firstly, I think this thread should mostly stay as a sticky about diversity ? !

For me the only two things technically (not suggesting it's not R-A-F, by the way!) would be not following manufacturers instructions (C3) and that length of .75 flex doesn't comply to the minimum size of 1mm IF we assume we classify the appliance as part of the fixed installation - again a C3 from me so long as there's no signs of degradation. There's nothing inherently wrong about installing something big fused by something small - we protect the cable not what's on the end of it - except it's likely to cause nuisance tripping. Again, from me that's only a C3.

 

[i=p/u]

Can go either way based on experiences C2 as worst case scenario. Work for social housing and change a few a day stinking of ---- and all charred.. but as said we protecting the cable , it just happens to protect the appliance too.

 

[Lucien Nunes]

@Rockingit I am curious as to why you don't consider the 0.75mm² cable to subject to significant overload. Here's my reasoning that it is.

To get a couple of things out of the way, I accept that the 13A BS1362 fuse protects it against short-circuit. I also accept that the long-term average current taken by the oven is likely to be very much less than 4400/230 = 19A maximum connected load, and that if the rating is 4400W at 240V the maximum load is 17.6A at 230V.

Looking first at the diversity, if we consider the oven and hob to be a single cooking station subject to the normal domestic 10A + 30% of the remainder, the total cooking load of 10.4kW gives a current after diversity of 20.6A split between the two FCUs. We had a thread recently about how that baseline 10A load should be divided between appliances if the oven and hob are separate. I maintained that the mere act of splitting a cooker into two pieces doesn't increase its load (subject to various detailed caveats about likely usage pattern) and I will stick to that.

If we gave the oven 5A of the baseline 10A load, it would require pretty much spot on 13A after diversity at 230V, and it is reasonable to calculate at 230V since under thermostatic control (which is assumed for an oven) the average current is determined by the prevailing voltage, not the rated voltage, (There is a very minor detail here about the impact of thermostat duty-cycle on cable heating, since for the same average heat input to the oven, a large element on low duty cycle heats the cable more than a small one on high duty cycle, but we can ignore this here as buried in the noise.)

Of course, diversity doesn't work like that, you can't arbitrarily divide the load into smaller units separately protected, because that undoes the averaging process at the heart of diversity. E.g. one might run four 10A loads on a 32A circuit, but one could not then assume to run two on a 16A. So we have to look at the load of the oven itself and its duty cycle and how that compares to the thermal time constant of the cable.

You switch on the ovens (perhaps for a pyrolitic cleaning cycle) and immediately it consumes its full load of 17.6A. This is 2.9 times the cable rating (6A for normal operating conditions for 0.75mm² 3183Y.) Hence the cable heat dissipation is (2.9)² = 8.6 times its maximum continuous rating, and it will very rapidly reach and exceed its maximum permissible conductor temperature. Obviously the rate of rise of temperature decays exponentially while the full load continues, but how does the thermal time constant of the cable compare with that of the oven, up to the first moment the thermostat opens? I don't have numbers, but I think it significantly shorter. Even if we give it the benefit of the doubt and consider the time constants equal, the cable will reach 0.63 x 8.6 = over five times its maximum permissible conductor temperature rise before the thermostat starts cycling.

While the usual cable failure mode associated with overload is premature ageing, in this case the temperature rise is sufficient to cause structural damage to thermoplastic insulation and to its terminations. Therefore although I suspect the OP's cable possibly still looks OK and might last years (and if they never switch on both the ovens simultaneously, it will be absolutely fine) nonetheless it is unstable and could escalate rapidly to a fire or shock hazard, hence a C2.

 

[Vortigern]

What was the IR result of the cables in question?

 

[littlespark]

If she's so adamant its safe, tell her to turn it all on at once and see how long it lasts.

Its a fire waiting to happen

 

[5544]

THANKS ALL, basically I was going between a C3 and a C2 so picked C2 after reading the manufacturers instructions. the well know appliance providers showed her the installation certificate, so she kept referring to that. In the end I walked away and did not post her certificate and hence did not ask for money. I dislike landlord work a great deal as they always go cheap and argue everything. (well where I am anyway). to make matters worse both of the ring mains (one which has the 6KW hob connected, and not on a ring. Someone has split both and they are basically radial circuits. Again pointed this out but in the end I have documented everything and walked away. She has found two electricians that have poo pooed what I have said so over to them.
I am a little happier reading your replies that I was in the right ball park. Some of the work I see these days is a real eye opener.

 

[telectrix]

THANKS ALL, basically I was going between a C3 and a C2 so picked C2 after reading the manufacturers instructions. the well know appliance providers showed her the installation certificate, so she kept referring to that. In the end I walked away and did not post her certificate and hence did not ask for money. I dislike landlord work a great deal as they always go cheap and argue everything. (well where I am anyway). to make matters worse both of the ring mains (one which has the 6KW hob connected, and not on a ring. Someone has split both and they are basically radial circuits. Again pointed this out but in the end I have documented everything and walked away. She has found two electricians that have poo pooed what I have said so over to them.
I am a little happier reading your replies that I was in the right ball park. Some of the work I see these days is a real eye opener.

with the split RFC/s that's got to be C2 all day long. you are right. they are wrong.

 

[Wilko]

Another vote for C2 from me. Possible upgrade to C1 if it’s already heat damaged.

 

[James]

Send her the invoice and certificate
gives you something to do in the evening, chasing someone for money.

they ordered an inspection, you have done it.
you should be paid, or at very least have a go at being paid.

don’t roll over so easy, it’s not your problem if they don’t like the result.

and for the installation certifixate from the installer. Not worth the paper it is written on.

 

[littlespark]

Yes agree, you still need paid. If she doesn’t like the result, that’s her look out.

Would like to know who these other electricians or engineers are that said it was all fine.

 

[James]

I can’t imagine either of these other electricians turning up to court if she wanted them to back her up that your report was fundamentally wrong.

 

[littlespark]

The “electrician” who is also a cab driver/ unregistered landlord/ tarmac layer and can get you a sky box with everything on it, no questions for 50 quid?

 

[Vortigern]

I think this is one of those areas where you can not leave it at that. I would issue the certificate and invoice. In the hope this would shake out the fact that LL has a potential fire hazard. I might even go so far as to inform the local council of the situation. I would certainly discuss this with my scheme. Bottom line you can't shrug your shoulders and walk away from it, you are leaving someone (tenant) in possible danger. Not within our remit to do!

 

[7029 dave]

I would also have issued a danger notice too, because it is a fire waiting to happen.

 

[Rockingit]

@Rockingit I am curious as to why you don't consider the 0.75mm² cable to subject to significant overload. Here's my reasoning that it is.

To get a couple of things out of the way, I accept that the 13A BS1362 fuse protects it against short-circuit. I also accept that the long-term average current taken by the oven is likely to be very much less than 4400/230 = 19A maximum connected load, and that if the rating is 4400W at 240V the maximum load is 17.6A at 230V.

Looking first at the diversity, if we consider the oven and hob to be a single cooking station subject to the normal domestic 10A + 30% of the remainder, the total cooking load of 10.4kW gives a current after diversity of 20.6A split between the two FCUs. We had a thread recently about how that baseline 10A load should be divided between appliances if the oven and hob are separate. I maintained that the mere act of splitting a cooker into two pieces doesn't increase its load (subject to various detailed caveats about likely usage pattern) and I will stick to that.

If we gave the oven 5A of the baseline 10A load, it would require pretty much spot on 13A after diversity at 230V, and it is reasonable to calculate at 230V since under thermostatic control (which is assumed for an oven) the average current is determined by the prevailing voltage, not the rated voltage, (There is a very minor detail here about the impact of thermostat duty-cycle on cable heating, since for the same average heat input to the oven, a large element on low duty cycle heats the cable more than a small one on high duty cycle, but we can ignore this here as buried in the noise.)

Of course, diversity doesn't work like that, you can't arbitrarily divide the load into smaller units separately protected, because that undoes the averaging process at the heart of diversity. E.g. one might run four 10A loads on a 32A circuit, but one could not then assume to run two on a 16A. So we have to look at the load of the oven itself and its duty cycle and how that compares to the thermal time constant of the cable.

You switch on the ovens (perhaps for a pyrolitic cleaning cycle) and immediately it consumes its full load of 17.6A. This is 2.9 times the cable rating (6A for normal operating conditions for 0.75mm² 3183Y.) Hence the cable heat dissipation is (2.9)² = 8.6 times its maximum continuous rating, and it will very rapidly reach and exceed its maximum permissible conductor temperature. Obviously the rate of rise of temperature decays exponentially while the full load continues, but how does the thermal time constant of the cable compare with that of the oven, up to the first moment the thermostat opens? I don't have numbers, but I think it significantly shorter. Even if we give it the benefit of the doubt and consider the time constants equal, the cable will reach 0.63 x 8.6 = over five times its maximum permissible conductor temperature rise before the thermostat starts cycling.

While the usual cable failure mode associated with overload is premature ageing, in this case the temperature rise is sufficient to cause structural damage to thermoplastic insulation and to its terminations. Therefore although I suspect the OP's cable possibly still looks OK and might last years (and if they never switch on both the ovens simultaneously, it will be absolutely fine) nonetheless it is unstable and could escalate rapidly to a fire or shock hazard, hence a C2.

@Lucien Nunes - You're quite correct and I stand, er, corrected. I didn't double check the ratings for 0.75mm and in my head thought it was slightly more generous than it is! I'm also reminded, when typing with brain engaged, that of course a 13a fuse won't ever disconnect at 4kw.