Max disconnection time

[dari_58]

hi lads i have few questions: what is max disconnection time for site hut lighting on a construction site ?
max disconnection time for 110 V reduced low-voltage system ?
reasons for the use of a 20 mA RCD ?
Thanks for your fast reply!

 

[IQ Electrical]

If the site hut supply is 230V then disconnection times are 0.4s on a TN system and 0.2s on a TT.

For 110V reduced voltage you have TN systems at 0.8s and TT at 0.3s

I think you might be reading an old question paper regarding 20mA rcd use as there is not actually such a requirement in BS7671:2008 but of course it could be used in situations of high shock risk such as school laboratory power supplies etc.

 

[malcolmsanford]

Are you sure it is 20mA and not 30mA

Most sites I've done will be TT and have an upfront 100mA RCD and then a CU with sockets on a 30mA and depending how the lights are installed they will have just a mcb supported by the upfront 100mA. Have a good look at sections 704 and 411.8 for reduced low voltages in the BS 7671-2208. Table 41.6 will give you the Zs times for the lights, but I'm sure they will be on some sort of RCD.

The 30mA and the 100mA will be 300mS @ x 1 and 40ms @ x5 if the system is TT then the X 2 for them is150mS maximum well within the 0.2 wanted.

If you using a time delayed 100mA then the times @ x 1 will be 130min/500max mS and @ x 5 will be 40min/150max mS