max allowable disconnection times?

Ryan Kavanagh · Mar 14, 2012

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

johnboy6083 · Apr 19, 2012

I also think (i may be wrong) that this takes into acount that a TT system will only be allowed upto a certain level, and after that then its either a TN sytem supplied, or the customer has HV supplied, and own their own transformer and associated network.

morph · Apr 19, 2012

johnboy6083 said:
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)

so why do the tolerances in TT discrimination not differ? from TN and surely the tolerances would need to be greater if the time was shorter

morph · Apr 19, 2012

to disconect more quickly the fault current would need to be greater thus the difference in protective devices would need to be greater surely ?

johnboy6083 · Apr 19, 2012

In TT systems 99.9% of the time, an RCD is needed to allow compliant disconnection. They can be co-ordinated, especially using S-types.

morph · Apr 20, 2012

Hi John

thing is RCD only disconnects L-E faults not L-N the CB does that so why is the disconection different when the same CB are used
so why do TT not have same disconnection time as TN
must be a logical reason , one reason could be the unstable parameteres as you mentioned.......

Guest123 · Apr 20, 2012

It does have a lot to do with the unstable parameters mentioned, such as poor earthing, lack of bonding etc.

Essentially the shock voltage difference between an TN & a TT system is roughly double, i.e a TN system with a good, reliable, low impedance fault path, with both line & CPC of the same CSA (disregard T+E here), would have a shock voltage of roughly 115V

TT would be almost as makes no difference be 230V which would travel through the installation & via the high impedance earth electrode, which is why we must reduce the disconnection time for clearance of an L-E fault.

I believe the 5s rule for TN systems was to allow the continued use of a 3036 re-weireable type fuse. It's reduced to 1s for TT to allow for discrimination to be achieved.