max Zs on a TT system for Nic eic

[daveygh]

Hi all

When filling in an NIC EIC for a TT system in the column maximum Zs permitted by Bs7671 do i put the values from table 41.5 (brb) ie 1667ohms if all the circuits are protected by BS61009 rcbo's regardless of the type?

thanks

 

[tom1]

i put 1667

 

[SPECIAL LOCATION]

Hi allWhen filling in an NIC EIC for a TT system in the column maximum Zs permitted by Bs7671 do i put the values from table 41.5 (brb) ie 1667ohms if all the circuits are protected by BS61009 rcbo's regardless of the type?

thanks

You are correct sir...

Read the title of the table..

It is exactly what you describe providing it is a final circuit NOT exceeding 32 amps.

And the whole section refers to TT installations see 411.5 up the page a bit

Guinness

 

[Lurch]

NICEIC is the organisation, EIC is the certificate.

 

[SPECIAL LOCATION]

NICEIC is the organisation, EIC is the certificate.

Factually correct Lurch...

:Salute

But I think that the OP is referring to the fact that 'NICEIC' Electrical Installation Certificates have the additional column for the 'Max Permitted Zs' that does not appear on the BS7671 model form EIC's!

GuinnessGuinness

 

[daveygh]

Factually correct Lurch... :Salute

But I think that the OP is referring to the fact that 'NICEIC' Electrical Installation Certificates have the additional column for the 'Max Permitted Zs' that does not appear on the BS7671 model form EIC's!

GuinnessGuinness

correct

 

[Lurch]

Don;t they? Well there you go, that's todays thing learnt.

 

[ianmacd]

so its an NICEICEIC?

 

[Lurch]

Well I know someone will be upset with the new acronym.

 

[daveygh]

just as an update to this thread can anyone tell me in the BRB the max zs for an a.c. unit on a 40amp circuit ? - the Nic inspector couldnt and also the guy who took my 17th edition course !

 

[kme]

`course they can`t tell you - you need to provide a little more info than that. OCPD would be a good start.

KME

n.b. I missed this thread first time round. I use 41.2 to 41.4 for max Zs; NOT 41.5. Never been commented on my AE.

KME

 

[daveygh]

`course they can`t tell you - you need to provide a little more info than that. OCPD would be a good start.KME

40 mcb 3 phase type c 60898 through a 30Ma rcd

n.b. I missed this thread first time round. I use 41.2 to 41.4 for max Zs; NOT 41.5. Never been commented on my AE.

KME

41.2 to 41.4 is for tn systems and none are for 1s disconnection times as 411.3.2.4

 

[kme]

But an MCB to 60898, with a Zs compliant with 41.3 gives disconnect of 0.4 AND 5 seconds; therefore 0.57 is the answer. What`s the problem?

 

[steptoe]

I think hes referring to a TT, 30 mA, so theoreticly 7666.66 ohms---------- Post Auto-Merged at 21:10 ---------- Previous post was made at 21:08 ----------

Though you also need to meet Ra x Idelta n < 50 which restricts to 1667 ohms

---------- Post Auto-Merged at 21:13 ---------- Previous post was made at 21:10 ----------

And if theres additional protection we need to do a 5 x test 30 mA - 150 mA for that we need it be less than 1553 ohms

---------- Post Auto-Merged at 21:15 ---------- Previous post was made at 21:13 ----------

But then were recommended to to have a Ra of 200 ohms BS7671, and 100 ohms 7430 so not really a issue

---------- Post Auto-Merged at 21:22 ---------- Previous post was made at 21:15 ----------

Oh and one other thing, when carring out a rcd test at 150ma(5 x 30) new meters will only allow upto 333ohms beyond this it will raise the voltage above 50v, so that limits it again.

is that actually a fact or an assumption for certain meters.?

only a question, I dont know so would like actual clarification on this before I quote it to someone.

 

[Chr!s]

is that actually a fact or an assumption for certain meters.?only a question, I dont know so would like actual clarification on this before I quote it to someone.

No its fact steptoe, you might get away with it on old meters but not new, think it may go in new GN3

 

[daveygh]

i'm not saying there was a problem, just asking where the table is

table 41.3 " maximum earth fault loop impedance for circuit-breakers with Uo of 230v, for instantaneous operation giving compliance with the 0.4s disconnection time of Regulation 411.3.2.2 (the maximum disconnection time stated in table 41.1 shall be applied to final circuits not exceeding 32A)and 5s disconnection time of regulation 411.3.2.3 (in a TNsystem a disconnection time not exceeding 5s is permitted for a distribution circuit no covered by regulation 411.3.2.2

mentions nothing of regulation 411.3.2.4 also if the Zs of 0.57 is not achievable in a TT system what alternative could you use?

 

[Chr!s]

also if the Zs of 0.57 is not achievable in a TT system what alternative could you use?

In most instances dave you will rely on your rcd for your TT fault protection

 

[daveygh]

i agree with that chris but on the test sheet it asks for max zs permitted by bs7671 and there isnt a table to back this up on circuits over 32A on a TT system

 

[steptoe]

No its fact steptoe, you might get away with it on old meters but not new, think it may go in new GN3

thanks plumber,

is this for all meters then?

I take it they are being designed like this now that when they reach 333 ohms they are just going to show >

makes you wonder how we are supposed to test for supplementary protection then.

 

[Chr!s]

is this for all meters then?

So im informed, product standards restrict voltages to 50v or less, so any test where for what ever reason will exceed the 50v the meter wont test

---------- Post Auto-Merged at 22:05 ---------- Previous post was made at 21:58 ----------

i agree with that chris but on the test sheet it asks for max zs permitted by bs7671 and there isnt a table to back this up on circuits over 32A on a TT system

Dave on a TT where RCD protection is used for Fault protection we have two requirements

Disconnection times need to meet either 411.3.2.2 or 411.3.2.4 and

Ra x idelta n < 50v.

So in the first instance we need to see what current needs to flow for the rcd to disconnect in the required time, from that we get our maximum Zs.

Then we have the 50V rule, so it will be the lesser of the two. hth