17th Edition Board on a TT system

[jl-heating]

never the less they fit them and ask us to so we do .

 

[Apache]

there are reasons to fit and reasons not to bother i would rather aire on caution and over protect. i only know we fit them and sometimes its a pain to actually sourse 1 but there you go we sleep at night knowing all bases are covered.

JL - can you name me 1 reason to fit a 100mA when everything is protected by a 30mA?

I can't think of one. There's no discrimination issue as the 30mA should trip first.

 

[jl-heating]

kieth says fit 1 so we do,

 

[Andy™]

kieth says fit 1 so we do,

and im telling you not to, so dont (but still charge the same price)

 

[green-hornet]

I have had similar discussions with councils who have a spec that is "wrong" but its like hitting your head against a brick wall.

I got told off for leaving blank plates over redundant but live cables ffs.

They wanted them all hidden.

 

[wozz]

JL are these 17th edition boards dual split with a couple unprotected ways? or just dual split?

 

[Evans Electric]

Around here you can get 15 to 35 ohms with a single rod no problem..I think the main benefit with a TT is that you are not relying in an old network for your earth connection..

Say you have a TNS supply (lead sheath) and the sheath rots... you now have a TT, but you don't know it's TT!!

Thats what is happening in this area, the cast joints are breaking down and the Ze,s are getting worse.

 

[Evans Electric]

TT can be better for domestic, but for commercial, its best avoided. saves the hassle of RCD incomers (since 99% of TPNE board are metal). and RCBOing a TPNE board isnt going to be cheap!).round here, its mostly TNS, newer are TNCS, but there are still a large number of TT, even in the built up area's

As far as I am aware ,Andy, commercial/industrial supplies always include an earth terminal.

Also there is no requirement for overall RCBO ,s on commercial/industrial .

 

[Andy™]

As far as I am aware ,Andy, commercial/industrial supplies always include an earth terminal.Also there is no requirement for overall RCBO ,s on commercial/industrial .

fortunately, no requirement. but if there was (and will most likely be in a few years), then it will start getting expensive

 

[Evans Electric]

Councils have their own agenda,s and , as someone has said, brick walls and head hitting comes to mind !!

I can remember two from the Birmingham standard spec.

Flush metal sheathing(capping) must be earthed with a 1.5 G/Y connected to sheathing with a self tapper.

All fixings will be Black Japanned round hd slotted woodscrews and fibre rawlplugs.

On a playing fields shower /changing rooms we were made to replace every screw because we'd used zinc plated PZ2,s C/Sunk.

 

[Nicky Tesla]

before the dual rcd boards, many ccts required their disconnection times like lights at 5s, but in tt systems this was often impossible without the use of an rcd. Being 5s a 100mA or even a 500mA rcd was adequate. Only such as socket ccts required the UPGRADE of the 30mA rcd for the 400mS trip time. If a large earth current was present, both the 100mA and the 30mA rcd could trip at the same time, being that they were in series.

The 100mA rcd is NOT required because the 30mA is an upgrade and on a split board the rcds are now not in series.

Im surprised anyone actually even has to think about this.

 

[davetheglitz]

Before the 17th edition , we used to put a 100ma Timed delayed RCD as the main switch to protect the lighting, cookers etc and a 30ma on the sockets.Now the 17th edition is out, 30ma on pretty much everything, my thinking is you dont need the 100ma Timed deley?

ohh...in a domestic installation of course

Is that correct?

many thanks

:Y

The only reason I can think of for a time delayed main switch is if you are feeding a consumer unit in a detached building. A 100mA time delayed RCD would be needed to ensure a trip if the cable was damaged as current flow to a rod would be insufficient to trip an MCB. Naturally you would have a 30mA RCD locally in the outbuilding. Result - a fault in the outbuilding does not trip the rest of the house and the outbuilding power can be easily restored.

 

[jl-heating]

JL are these 17th edition boards dual split with a couple unprotected ways? or just dual split?

dual split 17th edition boards with a couple of unprotected yes.

here lies the answer ty wozz.

 

[M107]

The only time I now use 100mA S types on a TT system are:

1. if the cu is metalclad & the 100mA goes on the tails
   
2. At the head of a submain supplying an outbuilding, if 30mA is used at said outbuilding.
   

Other than that no need to use 100mA S types if 17th ed dual 30mA cu's are used (not forgetting that circuits on non rcd ways on high integrity cu's need to be rcbo protected).

 

[davetheglitz]

The only time I now use 100mA S types on a TT system are:

1. if the cu is metalclad & the 100mA goes on the tails
   
2. At the head of a submain supplying an outbuilding, if 30mA is used at said outbuilding.
   

Other than that no need to use 100mA S types if 17th ed dual 30mA cu's are used (not forgetting that circuits on non rcd ways on high integrity cu's need to be rcbo protected).

Didn't think about the metalclad CU - thanks!

 

[Andy™]

Didn't think about the metalclad CU - thanks!

i said that back in post 14!

 

[M107]

Sorry Dave I only scanned through the posts quick earlier today...so well done matey

 

[davetheglitz]

i said that back in post 14!

Sorry mate - couldn't see the wood for the trees!

 

[Andy™]

Sorry mate - couldn't see the wood for the trees!

just goes to prove more that people here know what there doing!

 

[adyver]

I have just extended my house and my old memera 2000 box has insufficient ways for the new circuits. Not only that its a TT system with a 30ma RCD as main switch and protection on a solid board. (Fitted by the local authority before I bought the house). Consequently the whole system goes down if I get a nuisance trip. I intend to replace the CSU with an MK sentry 19 way box. It has a 100amp main switch and a I intend to fit an 80amp double pole RCD. I wish to protect a couple of circuits with RCBO's, if I get a nuisance trip on an RCBO is it also likely to trip the RCD, or can I put the RCBO's on the unprotected side of the RCD on a TT system.