adiabatic test

[ib60]

Hi all, I am new to this forum so apologies if this topic has been covered before!

Basically, I have been asked to test a commercial installation that is approx 2 years old however the original electrical contractor have gone bust. On reviewing the certificates I noticed that while 95% of the readings are correct there are about 40 circuits where the measured Ze exceeds the maximum permitted for the protection device installed, some are Type C RCBO’s and some are Type C Breakers. I know that they would pass if I changed all of them to Type B RCBO’s but this will be an expensive exercise.

Regarding the circuits protected by RCBO’s, I understand the RCBO still provides protection against shock risk in line with 411.4.9 and Table 41.5 but does not prove overcurrent protection in line with Chapter 43. I need to establish the L-L or L-N conductors are being protected against thermal damage under overcurrent conditions.

I think I can do this by showing that they pass the adiabatic test. It is a long time since my C course days and I cannot remember or find out how to do this therefore any assistance would be greatly appreciated.

If I can prove the circuits protected by RCBO pass the adiabatic test I intend to leave them and change the few circuits that are protected by the C Type MCB to a RCBO B Type.

 

[Sintra]

The adiabatic is only used to determine the size of protective conductors.

 

[telectrix]

the max. Zs figures refer to fault currents. not overload. basically, if you Zs readings are above the max. quoted in BS7671, then the OCPD will not disconnect in the stipulated time in the event of a fault of negligible impedance.

 

[Sintra]

As Tel says but they still be adequate for overload protection as per 433.1.1

 

[ib60]

thanks for your help lads is my only soloution then either to alter the circuit ie cable size etc or change to b type rcbo

 

[kingeri]

If Zs are not met and no RCD is in place, then yes, you either need to bring the Zs down or fit RCDs/RCBOs.

 

[ib60]

hi thanks but most of the circuits are alreadyprotected by c type rcbos so i need to change all 40 for b type rcbos is that correct

 

[D Skelton]

The adiabatic is only used to determine the size of protective conductors.

Not correct mate, sorry. There is more than one adiabatic equation

Hi all, I am new to this forum so apologies if this topic has been covered before!

Basically, I have been asked to test a commercial installation that is approx 2 years old however the original electrical contractor have gone bust. On reviewing the certificates I noticed that while 95% of the readings are correct there are about 40 circuits where the measured Ze exceeds the maximum permitted for the protection device installed, some are Type C RCBO’s and some are Type C Breakers. I know that they would pass if I changed all of them to Type B RCBO’s but this will be an expensive exercise.

Regarding the circuits protected by RCBO’s, I understand the RCBO still provides protection against shock risk in line with 411.4.9 and Table 41.5 but does not prove overcurrent protection in line with Chapter 43. I need to establish the L-L or L-N conductors are being protected against thermal damage under overcurrent conditions.

I think I can do this by showing that they pass the adiabatic test. It is a long time since my C course days and I cannot remember or find out how to do this therefore any assistance would be greatly appreciated.

If I can prove the circuits protected by RCBO pass the adiabatic test I intend to leave them and change the few circuits that are protected by the C Type MCB to a RCBO B Type.

The maximum Zs permitted for a 30mA RCBO regardless of type is 7667 ohms for TN systems and 1667 ohms for a TT system. The Zs is only measured in order to achieve compliance with fault protection only, ie to ensure that the OCPD disconnects the earth fault within the required time. Short circuits require no set disconnection time.

A Zln/ll higher than the max permitted for the overload element of an RCBO doesn't neccessarily mean that it won't protect the cable in the event of a short circuit. An adiabatic equation can be used to determine the thermal constraints of a cable to ensure that the overcurrent protective device or element of a device will trip before the cable burns out. It is a simple transposition of another well known adiabatic equation and is t = (s[SUP]2[/SUP] x k[SUP]2[/SUP]) / I[SUP]2[/SUP]. Using the current csa of live conductors, the correct 'k' value and the PSSC (normally) measured at the load point you can find the time it will take the cable to 'fry' under short circuit conditions. Cross examine that with your time/current tables in the BGB or even better manufacturers instructions you will be able to ascertain whether or not the OCPD will trip before the cable melts. I would very much doubt given the situation in your original post that any of the circuits are not adequately protected from short circuit or overload.

My thoughts:

Check that the r1+r2 values for each circuit are as low as they can reasonably expected to be or whether there are other factors at play which is reducing those results. If r1+r2 values suit the circuit then just change the remaining C 60898s for C 61009s.

Another option which does depend on the type of equipment being supplied would be to change the remaining C 60898s for B 60898s providing that in doing so you don't contravene any regulations regarding requirements for RCD protection.

 

[Welchyboy]

How much are they over by???

 

[i=p/u]

You can apply a formula to give you more accurate zs value too for circuit , maybe I'm thinking what d Skelton covered.... But il put my hands up if wrong and hopefully someone knows what I on abooot

 

[D Skelton]

You can apply a formula to give you more accurate zs value too for circuit , maybe I'm thinking what d Skelton covered.... But il put my hands up if wrong and hopefully someone knows what I on abooot

Just a wild guess but you're not on about correcting milliohms per meter values for ambient temperature are you? (1+(?x0.004))

 

[i=p/u]

Maybe d Skelton , but I remember something , but can't rember it. Is it where they use 5 times for breaker but the graphs is slightly different for required time + ambient temp???? Giving more accurate readings

 

[Silly Sausage]

You can apply a formula to give you more accurate zs value too for circuit , maybe I'm thinking what d Skelton covered.... But il put my hands up if wrong and hopefully someone knows what I on abooot

Are you thinking of the formula in GN3, Appendix A, A3, p115?

 

[D Skelton]

Maybe d Skelton , but I remember something , but can't rember it. Is it where they use 5 times for breaker but the graphs is slightly different for required time + ambient temp???? Giving more accurate readings

Ah, I think I know what you're on about.

5N, 10N and 20N for A,B and C type breakers retrospectively will give you a disconnection time of 0.1s but 6N, 12N and 24N is all that is required for a disconnection time of 0.01s?

Am I on the right track?

Edit: Thinking about it, maybe I'm not sure after all about the formula you're on about???

 

[Guest55]

some great psuedo physics going on here.
wake me up when you get to thermodynamics.
;-)