Regulation 434.5.2 exceeding limiting temperature of conductor.

[HappyHippyDad]

Evening..

I have a seperate oven and hob to fit.

The oven is 2.6kW, the hob is 3kW. I have fitted 4mm flex to the oven and the hob comes prewired with 2.5mm.

Zs on the cooker circuit is 0.2Ω making the PFC 1150A.

I was planning on connecting both to a dual cooker outlet plate fed by a 32A MCB (as reg 433.3.1(ii) shows that I can omit over current protection). However reg 433.3.1(ii) also states that it must be protected against fault current (hence reg 434.5.2).

In this example
t=0.1s
K=115
S=2.5mm
I=1150A

meaning that K²S² < I²t so the 2.5mm conductor may breach its permitted limiting temperature during a fault.

A couple of questions:

1. It would be common to have a 2.5mm radial circuit on a 20A MCB (I could even reduce the MCB on the above circuit to 20A) but if the Zs was 0.2Ω as above it would still not have adequate fault protection as K²S² < I²t??

2. If the Zs was higher (anything over 0.26Ω in the above example) then K²S² > I²t and the 2.5mm would not breach it limiting temperature in case of a fault, so it seems better to have a higher resistance!! Which doesn't make sense?

 

[uksparks]

Zs is 0.2? What's your Ze??

 

[PEG]

...I knew those sub-stations had kitchens in em' :wacko:

 

[Richard Burns]

You have taken your time as 0.1s however at 1150A the circuit breaker will trip in about 0.01s (according to most manufacturers time current curves)
If you were taking 0.1 s you should then choose the lowest current that can cause this disconnection time, 160A and be aware of the current limiting characteristics of the MCB so that you can take account of the actual higher current that may flow, because then only about 750A would flow and alter your calculation again..
However remember that the prospective fault current is just that, prospective and so will not be reached in most cases of an actual fault.

Ideally you would use tabulated values of the I²t for the MCB as this will give you the correct result.

I have not done this but I am sure that the cable would be protected. (*ready to be proved wrong!)

 

[HappyHippyDad]

Zs is 0.2? What's your Ze??

Ze = 0.17Ω. Guess the bonding must be pretty good! Any ideas on the question?

 

[HappyHippyDad]

You have taken your time as 0.1s however at 1150A the circuit breaker will trip in about 0.01s (according to most manufacturers time current curves)
If you were taking 0.1 s you should then choose the lowest current that can cause this disconnection time, 160A and be aware of the current limiting characteristics of the MCB so that you can take account of the actual higher current that may flow, because then only about 750A would flow and alter your calculation again..
However remember that the prospective fault current is just that, prospective and so will not be reached in most cases of an actual fault.

Ideally you would use tabulated values of the I²t for the MCB as this will give you the correct result.

I have not done this but I am sure that the cable would be protected. (*ready to be proved wrong!)

Thankyou Richard,

That all makes perfect sense. It always seems obvious when its explained well! Why would I think to use 1150A when the tables clearly show 160A for a disconnection time of 0.1s!

 

[Richard Burns]

I have now looked up some values of I²t for a 32A MCB (hager) and the values are less than 11,000 so compared to 82,656 it is a lot less, cable is protected.

 

[Deleted member 9648]

Ze = 0.17Ω. Guess the bonding must be pretty good! Any ideas on the question?

Ze test should not include the bonding, you are verifying the impedance of the means of earthing, extraneous conductive parts are not generally permitted as a means of earthing so should be excluded from the test.....PSCC/PEFC should include the bonding as parallel paths may reduce the impedance of the actual fault path and increase PSCC/PEFC.

 

[Risteard]

Ze test should not include the bonding, you are verifying the impedance of the means of earthing, extraneous conductive parts are not generally permitted as a means of earthing so should be excluded from the test.....PSCC/PEFC should include the bonding as parallel paths may reduce the impedance of the actual fault path and increase PSCC/PEFC.

Surely it only affects PEFC and not PSCC.

 

[Deleted member 9648]

Surely it only affects PEFC and not PSCC.

Depends on earthing type, would affect the PSCC on a TNCS/PME system.

 

[HappyHippyDad]

Ze test should not include the bonding, you are verifying the impedance of the means of earthing, extraneous conductive parts are not generally permitted as a means of earthing so should be excluded from the test.....PSCC/PEFC should include the bonding as parallel paths may reduce the impedance of the actual fault path and increase PSCC/PEFC.

I realise that Wirepuller, although my post was probably not clear enough! Zs = 0.2 and Ze = 0.17. I doubt the R1+R2 of the cooker circuit is 0.03, so I made a bit of a shot in the dark at the bonding being the reason!

 

[Risteard]

Depends on earthing type, would affect the PSCC on a TNCS/PME system.

Well with all earthing systems the neutral is earthed (otherwise it wouldn't be neutral), but point taken that with TN-C-S it may have a more severe effect on it.