TT sysyems

[iceman]

hi i was doin a minor works 2day on a light circuit on a tt system i couldnt get a reading for Zs cos me metre tripped out to determin Zs do i mesure Ze witch wos 38.8 and me R1 + R2 wos 0.61 = 39.41 this dont seem rite to me cos the tapulated value for a 6A breaker is 7.67 any help would be great cheers ;-)

 

[Wotsit]

Thats fine, maz value would be 1667ohms for your 50v touch voltage. Hence the need for an rcd on tt systems.

 

[iceman]

so im right by recording 39.41 for Zs aslong as theres an rcd in place if not id hav to put somethin the remarks collem sugesting action required?

 

[Wotsit]

Have a look at table 41.5 page 50 of BRB and reg 411.5

If there is no rcd for a socket circuit on a tt i use code 1 and for others i use a code 2 and if only one rcd is used for all circuits in a property i use code 4

I normally record the value i see although i beleive it is ok to record less than 1667

A value of 1667 is only ok if an rcd is used

 

[iceman]

im this case thn the rcd used was a 100A main switch with a 30mA trip so would state in the collem tht requires improvement

 

[Wotsit]

I would put code 4 - not inline with current regulations but not dangerous

 

[joe kane]

this is rght has to be less than 150ohms for ze thats why you ave to use a 100m/a time delay rcd

 

[Spudnik]

Ze has to be less than 1667 ohms.

Although anything over 200 is regarded as unstable.

 

[iceman]

nice one chaps thanks for ya help ;-)