Testing for ze on a submain cable?

[A-Non-eMouse]

hello chaps

i'm going to be doing some final live tests in a block of flats were doing at the mo on monday and was trying to remember what earth cable to test on for Ze in the flats consumer unit. The flats are fed by a 16mm t+e submain with an additional 16mm earth run along with it. now do i test for Ze using the t+e earth, for a worst case scenario, or the other 16mm earth. whats the correct procedure here?

also i didnt run the submains in but as i said earlier they are 16mm t+e plus additional 16mm earth. now as the run is over 3m am i right in saying that they have to be fed through a fused isolator as there not mechanically protected ie there not swa submains as currently they just come from the meter through a 63a double pole isolator and off to each flat.

thanks for your imput in advance

 

[Spudnik]

The tails must be protected by a switchfuse, suitably rated, or even an RCD if they are buried etc etc.

As for your Ze, measure both and record the worst, assuming they both will fall within limits.

 

[Davey101]

Personaly, I would treat the incoming position as an MCCB panel, with a continuation sheet filled out for it. Do your Ze at the origin, then do your R1+R2 on each sub main. Then just do a loop test at each board. I wouldnt worry about the integral earth, just test the 16mm, doesnt have to be disconnected from the db, cos you've already done your R1+R2 (properly). You cant really do a Ze at each board, just at the incomer. The result at each board is really a Zs (some like to call it Zdb).

Thats my take on it, and seems to be the accepted way of doing it.

 

[A-Non-eMouse]

cheers guys

just to clarify am i right in saying that if s.w.a was used then them being fused or having rcd protection can be omitted as it gives mechanical protection. also am i right to believe the additional earth can be ommited if run in three core s.w.a, banjoed at each end and each core correctly identified, ie green yellow tape, blue tape, brown tape.

thanks

 

[Guest123]

You will still need a suitably rated switchfuse at the origin, in the order of 60A say, to provide overload protection.

RCD protection isn't required and, yes you can use a 3core SWA to supply your L+N+E without an additional CPC, all correctly terminated and identified as you have said.

 

[Davey101]

cheers guys

just to clarify am i right in saying that if s.w.a was used then them being fused or having rcd protection can be omitted as it gives mechanical protection. also am i right to believe the additional earth can be ommited if run in three core s.w.a, banjoed at each end and each core correctly identified, ie green yellow tape, blue tape, brown tape.

thanks

I presume theres some sort of discussion (or argument) about how the sub mains are wired. A 3 core swa solves a lot of the problems, but there isnt anything wrong with T+E. Personally I'd want a switch fuse for each flat at the origin regardless of the wiring arrangents, easier for you, the flat occupiers, and any future sparks doing any work on them.
No need to banjo the Dboard end of the SWA.
Buy some sleeving-tapes a bit crappy on a decent size job IMO.

 

[Spudnik]

You should have a switchfuse at the origin if the tails exceed 3-4 meters in length.

The RCD would also be needed if the tails were buried etc etc.

 

[lloyd1980]

i remember this scenario a long time ago at college in the testing bays
there were 2 boards one was the main cu in house and another a cu in a garage fed by submain
we were told that take the submain as a circuit and take Zs reading then the new board (garage) as in what the submain feeds, take this as the origin on another test sheet and take the same reading at the same terminals only this time its the Ze of the sub board.
hope this helps
L

 

[Davey101]

Ze is external, it cant be measured at a dboard fed off a sub main.

 

[A-Non-eMouse]

so just to clarify,

check this out...........



DNO supply..........meter..............isolator...........flat consumer unit......end of radial circuit in flat

................................................ze here...........z's here or Zdb..........whats this called z's of z's?


hope this makes sense

 

[Guest123]

Ze would be measured at the sub-main isolator adjacent to the DNO service head.

At the flat CU you would measure Zs for the sub-main circuit

At the end of a radial circuit coming from the CU, lighting for example, you would measure the Zs for this circuit.

 

[Davey101]

so just to clarify,

check this out...........



DNO supply..........meter..............isolator...........flat consumer unit......end of radial circuit in flat

................................................ze here...........z's here or Zdb..........whats this called z's of z's?


hope this makes sense

Nope, doesnt make sense to me (but I am stoopid)

Lenny has pretty much covered it, but I'll try as well (in a very non technical, and half correct way)

The Ze is Z(external). This is the earth from the supplier, i.e. the lead sheath of a TNS, or the earth terminal of a TNCS, or the earth rod of a TT system. It is NOT an earth in your building, or at your DB (although you can usually measure it there). It can be measured at the incoming position, or at a DB connected to the mains, as long as you dissconnect the main earth from the DB, and main earths from the main earth terminal if there is one. In other words, your meter should be attached ONLY to the main earth cable, which is not attached to any other means of earth.i.e. pipework, or to any circuit earth. i.e. the earth bar in a Dboard. This should be done with the mains turned off.

Zs is, pretty much, any other loop reading. In other words, if you have more than one DB, or if the sole DB is not connected directly to the mains, then this is Zs. The loop reading on any circuit, no matter how its fed, is also a Zs.

 

[Bobby B]

not sure if this has been mentioned but you have to test with your main bonding disconnected to avoid false readings thru water and gas etc..