Downgrading MCB to protect cable but has fixed load greater than IZ

[gooner80]

Hi Guys

I know showers have been extensively covered on here, but this this is a slightly different question.

went to a job to replace 45amp shower switch, I noticed cable was damaged by heat, first thought maybe loose connection, so I started investigating, I noticed a 9.5kw shower on 6.00mm T&E cable enclosed in wall on 40 RCBO, I know without even looking that if it was clipped this might be ok but buried in wall no.

I explained situation to superviser and he was going to ask me to downgrade MCB. I was about to have a debate with him when I found further evidence of severe charring and at that point I said it needs rewiring, IR test showing L/E 0.14 M ohms.

My question is iz>=IN>=IB
downgrading MCB may cover IZ>IN but IB is not covered and my understanding is an MCB will aloow an overload of upto 1.45 times its rating in this case the current carrying capacity of the cable would be 34amps and the fixed load will pull more than this 46.4 for quite some time,thus damaging the cable, am I wrong? if I am in this situation again do I correct him?

thanks in advance for your help

 

[Andy78]

What sort of wall is the cable buried in and what derating factors have you retrospectively applied to the cable run ?

 

[gooner80]

What sort of wall is the cable buried in and what derating factors have you retrospectively applied to the cable run ?

It goes through loft insulation for 3metres that was enough for me, didn't check what kind of wall it was on vertical run down to consumer unit. My question is more about a fixed loaf with a design current in this case 9500watts/230 and the current carrying capacity of the cable being below Ib whether the mcb is downgraded this wouldn't offer sufficient protection?

 

[Andy78]

It goes through loft insulation for 3metres that was enough for me, didn't check what kind of wall it was on vertical run down to consumer unit. My question is more about a fixed loaf with a design current in this case 9500watts/230 and the current carrying capacity of the cable being below Ib whether the mcb is downgraded this wouldn't offer sufficient protection?

Ah I see. Your last post did not mention this, just that you thought burying the cable in a wall would drop it below 40A CCC.

The heat damaged cable in the switch is most likely down to a poor connection if it is only visible at the switch terminations, although an inspection and test of the cable would be a good idea if you suspect it has taken thermal damage along its length. (EDIT: missed your IR values in original post)

The circuit is clearly not suitable in its current state for the load.

 

[hightower]

L-E of 0.14 Meg ohms would surely trip the rcbo if the cable was as faulty as that. Was it tested with neons disconnected etc

 

[gooner80]

Hi I know how to apply rating factors my question is about Iz being lower than Ib and a mcb allowing 1.45 times overload

 

[gooner80]

L-E of 0.14 Meg ohms would surely trip the rcbo if the cable was as faulty as that. Was it tested with neons disconnected etc

Ir was on load side disconnected. the switch was damaged thus no trip.

 

[Midwest]

I think unless you want to get into an academic argument with your supervisor, from you IR test it appears the cable is faulty and needs replacing. Probably because, as you have identified that the cable is too small in the circumstance you described.

I suspect the 9.5kw shower is a replacement for smaller previous shower. There will be a few will say that a 6mm cable, could safely carry this load, unless the occupants were having 1/2 hour showers (but that might be the case), and a combination of 6mm cable with 32a ocpd would be able to carry 9.5kw load for sufficient periods.

In any case, if it was indeed loose connections that have caused the thermal damage, then a good section of the cable will need to be removed, and the supply cable might have to be replaced. If that is also the case, then a correctly sized cable & ocpd would need to be installed

 

[westward10]

I would also guess the damage is caused by loose connections. The situation you have I would say the cable needs upgrading.

 

[gooner80]

Yes I recommended to either change shower or upgrade the cable my poin was I was about to have a debate about downgrading rcbo wouldn't be sufficient. If cable wasn't damaged

 

[Darkwood]

Reg' state 433.1 a circuit should not be designed where a small overload can occur for a long duration.. the definition of long duration is a grey area but I suspect it is ambiguous because some loads do have inrush currents and occasional overcurrents due to the nature of the load IE a fridge motor switching on and off several times a day.
A shower is a fixed load so having a ocpd higher than the cable rating is not an issue but at the same time it's not generally a practiced design method, although the cable must be large enough to handle the current of the shower with all factors accounted for in its installation methods.

You need to refer to 433.1 as mentioned above and 433.1.1 to build your case here.

 

[Midwest]

Yes I recommended to either change shower or upgrade the cable my poin was I was about to have a debate about downgrading rcbo wouldn't be sufficient. If cable wasn't damaged

I agree with you. What would your design be for the new install?

 

[Lucien Nunes]

If the MCB were small enough to protect the cable, and yet large enough not to trip under normal usage, then that usage would not have damaged the cable. Or, put another way, if the cable isn't large enough to carry the load, no size of circult breaker will protect it but not trip. All it could possibly do is to limit the length of showers to a duration that won't damage the cable, which will see a steady stream of angry wet people trekking to the CU to reset it!

Ignore the fusing factor of the MCB, that is taken account of in Iz. Giving a cable an Iz of 32A means it can be adequately protected by an EN30898 MCB of 32A rating (including its fusing factor), not that the cable can only survive an absolute maximum of 32A.

L-E of 0.14 Meg ohms would surely trip the rcbo

Why 'surely'? 0.14MΩ suggests a badly damaged cable and the IR could be variable and all sorts of nasties might be happening, but 0.14MΩ itself won't trip the RCD. 230/140,000=1.6mA. If that resistance is at 500V then it might be higher at 230 and not leak as much. Actually if there is a PVC cable with 0.14MΩ it will be interesting to see it.

 

[gooner80]

If the MCB were small enough to protect the cable, and yet large enough not to trip under normal usage, then that usage would not have damaged the cable. Or, put another way, if the cable isn't large enough to carry the load, no size of circult breaker will protect it but not trip. All it could possibly do is to limit the length of showers to a duration that won't damage the cable, which will see a steady stream of angry wet people trekking to the CU to reset it!

Ignore the fusing factor of the MCB, that is taken account of in Iz. Giving a cable an Iz of 32A means it can be adequately protected by an EN30898 MCB of 32A rating (including its fusing factor), not that the cable can only survive an absolute maximum of 32A.



Why 'surely'? 0.14MΩ suggests a badly damaged cable and the IR could be variable and all sorts of nasties might be happening, but 0.14MΩ itself won't trip the RCD. 230/140,000=1.6mA. If that resistance is at 500V then it might be higher at 230 and not leak as much. Actually if there is a PVC cable with 0.14MΩ it will be interesting to see it.

If I'm asled to take it out ill show you I have a pic of part of the burnt cable that I pulled down when visually insolpecting

 

[gooner80]

Sorry typing on phone is quite hard I should really proof read what I write that should have been if I'm asked to rewire I can show you. It's interesting the number of sparks that are defensive of 9.5kw on 6.0mm. If it is designed and calculated right that's fine. Me personally I err on the side of caution, I've been to a few house fires on emergency call out. This installation goes through insulation in the attic as mentioned before I know how to apply rating factors . It seems it is a grey area as mentioned by Lucien, if I had to go to court over something like this I would rather be on the right side of grey area