max allowable disconnection times?

[Ryan Kavanagh]

hi guys,

what is the max allowable disconnection time times under fault conditions for kitchen lighting, bedroom socket outlets and also for a shower?

thanks in advnace i appreciate all your help

 

[Ryan Kavanagh]

ive seen in the 17th edition it says 0.4sec. is this right across the board for all the circuts.

 

[Ryan Kavanagh]

sorry just re read yet it is for all final circuts not exceeding 32a

 

[a1guvner]

sorry just re read yet it is for all final circuts not exceeding 32a

Have another look

 

[telectrix]

it is 0.4 secs. for circuits up to 32A. 5 secs. for circuits over 32A and didtribution circuits.

 

[IQ Electrical]

it is 0.4 secs. for circuits up to 32A. 5 secs. for circuits over 32A and didtribution circuits.

Assuming this is a TN system.

 

[telectrix]

thank you IQ. 0.2 secs,and 1 sec respectively for TT.

 

[Ryan Kavanagh]

yes thats what i read but i thought tn systems were most common for domestic uk supply thats were i came to the 0.4sec conclusion.

 

[telectrix]

TN systems are more common, but you will get plenty of TT especially in rural areas.

 

[IQ Electrical]

yes thats what i read but i thought tn systems were most common for domestic uk supply thats were i came to the 0.4sec conclusion.

It very much depends on where you are, in my neck of the woods (Northumberland) I'd say it's about 50/50 TN to TT.

 

[Ryan Kavanagh]

okay guys, you have helped me alot with getting to grips with it.

thanks

 

[morph]

TT is 1 second
TN is 5 second
question is why are they different?

 

[Guitarist]

TT is 1 second
TN is 5 second
question is why are they different?

Electricity travels faster overhead as the ground doesn't get in the way. Therefore, the same amount of power can be transferred in 1 second with a TT system as gets transferred in 5 seconds via a TN setup.
Hope this clears it up for everyone.

 

[morph]

Electricity travels faster overhead as the ground doesn't get in the way. Therefore, the same amount of power can be transferred in 1 second with a TT system as gets transferred in 5 seconds via a TN setup.
Hope this clears it up for everyone.

:13: genius :clap:

 

[johnboy6083]

the 5 second rule for TN systems is set at such a level to allow discrimination between protective devices. To have distribution circuits any lower would mean that the tolerances for discrimination would need to be very tight indeed. The earthing on a TN system is said to be reliable and constant, where as a TT system is not. Also prolonged faults in a TT sytem can cause the ground potential to rise, giving a shock risk (the dead horse a few months ago)