17th edition testing
- Thread starter sijaysee
- Start date
All final circuits up to 32A on a TN system now have a reduced 0.4s disconnection time.
Would that 0.4 second disconnection time Apply to highway power supplys too ?
No, all fixed equipment used in highway power supplies have a maximum disconnection time of 5s (559.10.3.3).
RTFB (Read The 'effing' Book)
If you don't know the regs how can you comply with them? And if you are doing this work for profit, not having a copy is practically criminal!
0.2s IIRC. Will check this evening as ive not got BS7671:2008 with me.
Garymo, chock on lad, Quote the Red Bible
Maximum disconnection times
120V <Uo =<230V TN 0.4 max TT 0.2 (a.c.)
230v < Uo =<400 TN 0.2 TT 0.07 (a.c.)
This leads to another question however
When regularly reading the supply voltage I get over 230v normally 249v ish
Now I was always told to use the readings YOU get, so does that mean the latter should really apply, or lets just all agree it is 230v and so the former applies
Maximum disconnection times
120V <Uo =<230V TN 0.4 max TT 0.2 (a.c.)
230v < Uo =<400 TN 0.2 TT 0.07 (a.c.)
This leads to another question however
When regularly reading the supply voltage I get over 230v normally 249v ish
Now I was always told to use the readings YOU get, so does that mean the latter should really apply, or lets just all agree it is 230v and so the former applies
No, it's a standard 230v supply so use disconnection times accordingly.
When regularly reading the supply voltage I get over 230v normally 249v ish
Now I was always told to use the readings YOU get, so does that mean the latter should really apply, or lets just all agree it is 230v and so the former applies
As GaryMo has indicated, this is the nominal line voltage to earth, Uo, so the times stated in table 41.1 apply.
There is nowhere on any BS 7671 form to record the actual voltage at consumers' supply terminals, so I'm not sure who has told you to use such readings and for what purpose.
Hi Dingbat, what was being suggested by the lectures at college was that although there is a nominal figure of 230v, we know that often we can find voltages in excess of this.
After all, they've really only changed the nominal voltage in number rather than supply thus far.
So the arguement is that when investigating faults / new installations etc that it may also prove beneficial, in some cases, to carryout calcs using the actual voltage to ascertain if this could be having any adverse effects.
Thats all
After all, they've really only changed the nominal voltage in number rather than supply thus far.
So the arguement is that when investigating faults / new installations etc that it may also prove beneficial, in some cases, to carryout calcs using the actual voltage to ascertain if this could be having any adverse effects.
Thats all
But you were asking about the application of the wiring regulations, in which only nominal voltages are used.
An example would be maximum earth fault loop impedance for a given protective device, lets say a BS60898 B6 MCB......
Using 230v: 230/(6x5) = 7.66ohms (6.13ohms 80% ROT)
Using 249v: 249/(6x5) = 8.3ohms (6.64ohms 80% ROT)
Basically, using 230v would be erring on the side of caution and what you should base your cable calculations on.
Using 230v: 230/(6x5) = 7.66ohms (6.13ohms 80% ROT)
Using 249v: 249/(6x5) = 8.3ohms (6.64ohms 80% ROT)
Basically, using 230v would be erring on the side of caution and what you should base your cable calculations on.
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