Disconnection times

[La Poste]

This might help a few of us suffering the 2391 exam.

Maximum disconnection times for a nominal rms line voltage to earth, U0 of 230V

Firstly for a TN system:

Final circuits not exceeding 32A = 0.4s
Final circuits exceeding 32A = 5s
Distribution circuits = 5s

For a TT system

Final circuits not exceeding 32A 0.2s
Final circuits exceeding 32A = 1s
Distribution circuits = 1s

A maximum disconnection time of 5s applies to all circuits in a reduced low voltage system.

A maximum disconnection time of 5s applies to all circuits supplying fixed equipment used in highway power supplies.

I'm still not sure about electrically separated circuits.
I mean if we have a 50V SELV circuit and we get a line to neutral fault on the low voltage side does this mean the disconnection time is 5s?
Seems a bit long to me.
There seems to be a lack of clear guidance when testing electrically separate circuits for insulation resistance and disconnection times.
Anyway, hope this table helps.

Just one more question.
What is the logic behind a five second disconnection time.
Why is it some circuits are allowed a five second disconnection time and others not?

 

[SirKit Breaker]

This might help a few of us suffering the 2391 exam.

Maximum disconnection times for a nominal rms line voltage to earth, U0 of 230V

Firstly for a TN system:

Final circuits not exceeding 32A = 0.4s
Final circuits exceeding 32A = 5s
Distribution circuits = 5s

For a TT system

Final circuits not exceeding 32A 0.2s
Final circuits exceeding 32A = 1s
Distribution circuits = 1s

A maximum disconnection time of 5s applies to all circuits in a reduced low voltage system.

A maximum disconnection time of 5s applies to all circuits supplying fixed equipment used in highway power supplies.

I'm still not sure about electrically separated circuits.
I mean if we have a 50V SELV circuit and we get a line to neutral fault on the low voltage side does this mean the disconnection time is 5s?
Seems a bit long to me.
There seems to be a lack of clear guidance when testing electrically separate circuits for insulation resistance and disconnection times.
Anyway, hope this table helps.

Just one more question.
What is the logic behind a five second disconnection time.
Why is it some circuits are allowed a five second disconnection time and others not?

You need to look at the tables in the BRB, whilst your info on the face of it looks correct, you have not accounted for the type of protective device in your disconnection times. If the protective device is a MCB to BS60898 the maximum disconnection time is 0.4 seconds irrespective of the type of circuit, There are other devices as well such as 61009, 61008 and so on. I don't wish to dampen your enthusiasm but this is a classic case of "You need to read the question properly".

Cheers.............Howard

 

[La Poste]

Thanks for the advice.

The table was meant as a general guideline for instance there was a question in a previous test paper which was worded "what was the maximum permissible disconnection time when testing a BS EN 300ma RCD supplying outside lights with a TT earthing system."
Now on the face of it a 300ma BSEN RCD would have a disconnection time of 300ms but since it is used to supply a circuit in a TT system then the disconnection time has to be 0.2s which is 200ms.
This question caused quite a lot of confusion in a previous exam.

 

[Deleted member 9648]

The logic behind a 5s disconnection time is that the risk of contact and hence shock risk in the event of a fault is lower with some circuits than others.........socket outlet,handheld portable appliance = high risk
distribution circuit serving DB = low risk of contact during fault.

 

[IQ Electrical]

Thanks for the advice.

The table was meant as a general guideline for instance there was a question in a previous test paper which was worded "what was the maximum permissible disconnection time when testing a BS EN 300ma RCD supplying outside lights with a TT earthing system."
Now on the face of it a 300ma BSEN RCD would have a disconnection time of 300ms but since it is used to supply a circuit in a TT system then the disconnection time has to be 0.2s which is 200ms.
This question caused quite a lot of confusion in a previous exam.

The answer is still 300mS for the device.

Check out Note 2 at the foot of Table 41.1 the 0.2S disconnection time is for fault currents considerably higher than I delta n.

Then check out Appendix 3 values for RCD disconnection times even for as little as 2X I delta n.

 

[La Poste]

I agree with you but I also read the examiners report on the exam and he said that people quoted the disconnection time of 300ms for the circuit and this was wrong as the question specifically stated it was a circuit supplied by a TT earthing system.
You remember the question I am sure you do it was to do with a marina, it was one of this years papers.
I have not seen the exam paper from from the sounds of it it was a trick question.
The question was asking you what the acceptable disconnection time of this circuit would be if it was supplied by a 300ma or 500ma RCD I forget which.
People just looked at the RCD and quoted 300ms.
The disconnection time should have been written as 200ms.
I have read the foot note you talk about but I think to expect people to remember all this, the 2 times I delta n figure for example, for a 2391 exam is a little unfair.

 

[IQ Electrical]

I agree with you but I also read the examiners report on the exam and he said that people quoted the disconnection time of 300ms for the circuit and this was wrong as the question specifically stated it was a circuit supplied by a TT earthing system.
You remember the question I am sure you do it was to do with a marina, it was one of this years papers.

I said 'the device', the disconnection time for the circuit, assuming it wasn't a distribution circuit would be 0.2S.

 

[La Poste]

And that's why I put these tables up in case we have another question like it.
I guess it all boils down to the wording.
As you say:
"What's the maximum disconnection time of the device = 300ms
"What's the maximum disconnection time of the circuit = 200ms.

Trick question.