TNS to TT

[mikep]

I discussed a problem last week on here about a customer in the next street to mine with the same house, built in 1934 which was rewired 15 years ago with the old Wylex wire fuses, mine has TNS fitted but with 16th edition , I am to fit a new boiler and done the walk round but noticed the 16mm was never connected to the outer Armour of the cable of the incomer and that it has never been had an earh strap fitted since new in 1934.

This has been relying on the services for earthing, I rang EDF they said that they would charge to TNS, or TNCS it.

on this forum I was advised just to TT it and now that makes sense to me.

Old couple with a shot boiler and not a lot of money.

As a lot of you guys have a lot more experience of conversion to TT than me, your advice would be appreciated.

Do I Bang a rod in, test the Ze test it all and make sure that it meets the correct Zs readings on all circuits for disconnection times and Rcd my boiler, then again fixed Equipment does not have to be RCD'd.

Should it be a new Board with twin 30ma RCD or what as TT is quite grey area to me

Don't want to go to over the top but it's got to be right.

Look forward to your views on this.

Mike.

 

[Apache]

AFAIK All circuits must be RCD protected on TT

Isn't it Ra not Ze on a rod?

 

[Andy™]

Stick rod in an test. patch is right on the RCD - everything on TT must be RCD'd. just remember your Zs on TT can be upto 1667 ohms assuming 30mA RCD although for your rod, anythng above 200 should be considered unstable

Ra is rod impedance, but Ze applies to impedance external to the property, so Ra/Ze is the same

 

[mikep]

Yep you right about RA, it was one question in the exam a couple of months ago, gave 3 impedances and asked for the RA, I assumed it was the impedance in parrallel formula, laid it out all nicely thought I had that question cracked then realised when I got home that I gave the wrong answer.

Anyway so what is the best way out and the cheapest for them, Board change !

I think there's a transformer in the next road so should be ok.

Might just run a 16 mm to it HaHa.

 

[Andy™]

you could always pull cable from DB, install into external RCD+ case and cable between that and fuse/mcb. or change to RCBO. or stick RCD in tails

 

[mikep]

Well I was going to suggest Rcd the tails, might be the cheapest way with a rod. does that comply.

 

[Andy™]

kinda... a problem on one circuit should effect another, so an RCD protecting more than 1 circuit doesnt comply, but its more cost over inconvenience, so for how often its going to trip, its not normally a major problem. that reg hasnt changed for many years, and there are many houses with one RCD on tails and never a problem

 

[Apache]

kinda... a problem on one circuit should effect another, so an RCD protecting more than 1 circuit doesnt comply, but its more cost over inconvenience, so for how often its going to trip, its not normally a major problem. that reg hasnt changed for many years, and there are many houses with one RCD on tails and never a problem

unless we have an existing problem somewhere

 

[Andy™]

unless we have an existing problem somewhere

yes, but even if you do have an existing problem and all RCBO's, its still going to trip! and if there are shared neutrals, it wont trip a main RCD but will trip RCBO's!

 

[mikep]

Well after looking at RCD's it looks cheaper to renew the CCU but then the headache of going through the existing circuit faults to rectify.

 

[Andy™]

i rarely have any major problems after changing a DB. maybe get the odd low IR (<10Mohms), but rarely anything that causes RCD to trip

 

[mikep]

how far are you suppose to go with regard to renewing CCU, do you go round stripping and checking every point etc to make sure compliance to 17th or do you just test each of the circuits at the board and if they pas you connect them, never seen any paperwork on that for guidance!

 

[Andy™]

check that circuit is safe (IR, Zs etc), fill in EIC and go to next job/home

 

[mikep]

That right you don't have to strip or inspect anything just test circuits, be fantastic if anyone can let me know any reading on this subject

 

[mikep]

Just thought the regs do say that it should not be left in any worse state.

 

[Andy™]

if they want a full PIR, its a seperate job. just do the basic checks to make sure its safe and your done. there wa a thread on the not too long ago - im sure patch can find it

 

[mikep]

Sitting here waiting for a reply, did'nt see there's another page to turn.

Many thanks to you and apache,

As Benny Hill Use to say "Learning all the time"

 

[dianemil]

The electricity supply act states that if a Ze is available from the suppliers incoming facility it msut be provided to the customer.

strip a bit of the bitumen covering of the lead and do a Ze test, if its good enough you can insist the supply is altered, and i think it became unchargable in 1988.

 

[green-hornet]

The electricity supply act states that if a Ze is available from the suppliers incoming facility it msut be provided to the customer.strip a bit of the bitumen covering of the lead and do a Ze test, if its good enough you can insist the supply is altered, and i think it became unchargable in 1988.

It is never a good idea to interfere with any of the supply cables.

It still is an offence and you could face prosecution for doing it.

Removal of any of the outer coating to reveal the lead sheath could be dangerous without some training and knowledge.

I can understand your logic but it is never a good idea to post something on a forum that is illegal under the present allthough never enforced laws.

 

[NozSpark]

The cheapest way to do this would be to stick a rod in, upgrade the PEB's if necessary, split the tails and add a new RCD CU for your circuits ..... note on EIC that other circuits from other CU have no RCD protection..... Issue danger notice for old CU and get client to sign for receipt.

Not the best way, but certainly the cheapest!!