conduit ring circuit

[martinh287]

Hi guys after advice

Large old manor hall wiring 30 yrs old. Tn-c-s Few faults rectified and board changed to 17th edition.

Downstairs orginally had a ring wired in steel conduit and singles. Using the conduit as a earth. Look like a door way was put on to make a extension to the house a few years ago and in doibg so a socket was removed and jb put in its place. The conduit has been bent down to below ground level and it is at this point where the ring is no longer a ring.

I split the ring on to 2xb20

Leg 1 = 0.58 r1+r2 0.83 zs
Leg 2 = 45.6 r1+r2 45.8 zs

The final socket on leg 2 actually gets 0.61 and 0.86 but every other socket back to the board is in the 45 ohm range.

The house is registered with english heritage and the owner is looking to sell. Lot of expense rewiring.Tried running new earth in the conduit but to no avail.

The garage is fed from a fuse spur on this radial. I was thinking bout dropping a rod external to the garage and on the the db board in the garage. This should lower my zs on all but one of the sockets. ( assuming I am correct with my evaluation of where a break in the conduit seems to be)

Am I right in thinking there is nothing wrong with dropping a rod as suggested or anyone any other suggestions.

 

[KAS1]

I'd make more enquires as to who left it in that condition and I'd hunt them down, then drag them back to re-instate the conduit to complete the ring (earth connection is paramount), then explain to the owner of the building what a dangerous state it had been left in.

 

[HandySparks]

The conduit has been bent down to below ground level and it is at this point where the ring is no longer a ring.

I split the ring on to 2xb20

Am I right in thinking there is nothing wrong with dropping a rod as suggested or anyone any other suggestions.

So, is the conduit itself in the form of a ring (ie two ends connected to the DB), or is it just the circuit that was wired as a ring?

Why did you split the ring?

Using earth rods to make up for a break in CPC sounds like a bodge to me.

 

[johnboy6083]

You wouldn't improve the Zs one bit with a rod. What you'd effectively be doing is making the circuit TT.
theoretically, you'll already be covered for the high Zs as its on an RCD (or should be)
how do you know it's high impedance on the CPC? Have you got end to end readings, and R1 + Rn readings?
This is a difficult one to remedy properly due to English heritage. They won't want surface mounted trunking/conduit.

 

[trev]

I don't understand why you're talking about dropping rods in, there'll still be a break in the cpc of this RFC.

 

[martinh287]

High zs values are accepted on a tt system with both 30mA and 100ma in the hope that in the event of a failure of one rcd a .4 disconnection time can still be achieved.

Putting a 100ma rcd time delayed before the board at source is a option. I dont believe dropping a rod at source and changing to a tt is making the system safer even thou the system would then comply.

 

[trev]

It's not a TT though Martin it's a TN-C-S.

 

[martinh287]

I know its tn c s and as it stands it doesnt comply. But add 100ma rcd and make it tt and and it does.

My argument is adding the 100ma rcd makes sense with valid reasoning but removing the supplied earth and providing a higher ze for the rest of the circuits by putting in a rod is making the installation more unsafe then just adding 100ma rcd and keeping the lower provided earth path

I hope you can understand my thinking

 

[trev]

Yeah I understand you're thought but even with adding rods and making it a TT there's still a break in the cpc of this RFC and no amount of adding the magic RCD will fix that.

 

[SPARTYKUS]

If its conduit cant you sytematically pull a new line in until you fix it?