RCBO's on TT

[Oracle]

All,

Red book and OSG give max Zs for RCBO's, but what happens on a TT installation when Ze could be upto 200 ohm?? This would blow the max permitted Zs with the RCBO - or isn't it important as long as Zs/Ze is less than 200 ohm?

O.

 

[Andy™]

All,Red book and OSG give max Zs for RCBO's, but what happens on a TT installation when Ze could be upto 200 ohm?? This would blow the max permitted Zs with the RCBO - or isn't it important as long as Zs/Ze is less than 200 ohm?

O.

because it contains an RCD, Zs can be upto 1667 ohm. (assuming 30mA tripping current)

(But any Ze higher than 200 should be considered unstable)

thing these days to watch for is now L-N loop

 

[SPECIAL LOCATION]

All,Red book and OSG give max Zs for RCBO's, but what happens on a TT installation when Ze could be upto 200 ohm?? This would blow the max permitted Zs with the RCBO - or isn't it important as long as Zs/Ze is less than 200 ohm?

O.

I assume the tables you are referring to are those which relate to TN systems.

i.e. within section 411.4 of the big red book and Appendix 2 OSG.

you need Reg 411.5.3 & table 41.5 which refers to TT installations.

pg 50.

 

[binky]

If it makes more sense, I look to achieve normal Zs results for circuits within the building + Ra for the rod (upto 200ohms). Otherwise you could have 199ohms on your circuit and 1ohms for the rod, which is not good, even if it reads as being legal.

 

[NozSpark]

I do the same as Blinky..

 

[binky]

I do the same as Blinky..

That's a 3 eyed fish (simpsons)

 

[SPECIAL LOCATION]

If it makes more sense, I look to achieve normal Zs results for circuits within the building + Ra for the rod (upto 200ohms). Otherwise you could have 199ohms on your circuit and 1ohms for the rod, which is not good, even if it reads as being legal.

Indeed so MR binks!Guiness DrinkB-)

That is what we call.. Common bloomin sense!!!

or GOOD working practice IMHO!Applaud Smiley

 

[steptoe]

remember tho, on a TT your RCBOs MUST be DP, and not the regularly used SP ones.

SP RCBOs with no other protective measure in place would consitute a major code 1 IMHO.

HTH.

 

[Andy™]

even more reason to use a dual split...

 

[batty]

remember tho, on a TT your RCBOs MUST be DP, and not the regularly used SP ones.SP RCBOs with no other protective measure in place would consitute a major code 1 IMHO.

HTH.

Why would this be Steptoe.

Batty

 

[Oracle]

i like getting people to think

On tt systems there is a requirement for all live conductors to be disconnected under a fault conditions - domestic rcbos are single pole (normally) - whereas rcds are dp.

So a high integ board would not comply for the circuits that hang direct off the isolator via rcbos,

O.

 

[Andy™]

on TN, the neutral can be assumed to be at earth voltage, so you can use SP RCBO's. on TT, neutral may be different from earth, so has to be switched. or something like that

 

[batty]

i like getting people to think On tt systems there is a requirement for all live conductors to be disconnected under a fault conditions - domestic rcbos are single pole (normally) - whereas rcds are dp.

So a high integ board would not comply for the circuits that hang direct off the isolator via rcbos,

O.

The neutral is not a live conductor. You only need to disconnect the live and neutral at the mains so a double pole switch which all main switches are will be adequate. Only caravan parks and some other installs require double pole RCD switching.

 

[Andy™]

Both line/phase/whatever you want to call it, and neutral are 'live' conductors

 

[steptoe]

The neutral is not a live conductor. You only need to disconnect the live and neutral at the mains so a double pole switch which all main switches are will be adequate. Only caravan parks and some other installs require double pole RCD switching.

I think you will find that it is, both in the big red red comic and in real life.

lets go back to basics now,

disconnect your neutral from your lighting cct(for example),

switch on a light ,

now test between neutral and earth,

and you still want to say the neutral is NOT a live conductor?

what happens on the DNO side of the cut out(where neutral goes to earth potential at the transformer) is of no consequence to you under normal circumstances.

 

[M107]

Funny enough I called the nicey tec dept today on just this subject (using a TT system with rcbo's as assessment).

Their view is

"Type C or any type of rcbo are fine on a TT system even if single pole."

when I question the need for dissconnection of line & neutral under fault conditions.

"No the single pole rcbo option is viable as all you are wanting is dissconnection of line conductor"

"using rcbos meets all the requirements of the regulations"

I then asked would it not be an option to use a S type as main switch then rcbos.

"why when the use of 30mA rcbos meets the requirements for a TT supply, anyway the s type couldnt be classed as a saving life device"

So there you have it rcbos are ok..........................now on my assessment in a few weeks if I get picked up for using type C single pole rcbo's I'm gonna kick up one big s*** storm on the tech line.

 

[Evans Electric]

Lets get this clear. Are we saying that on TT systems , double pole RCBO,s must be used , or not, because I did not know this was the case .

Deke

 

[M107]

Same here Deke hence the call to niceic.

Well nicey say no it's not a requirement & when I said what about dp isolation the chap said well you'll have the main switch, but DP isolation is not needed on the rcbo's.

I feel dual boards comming on for TT's to many interpretations on the use of rcbo's.

 

[Evans Electric]

I also lean towards dual boards but mainly due to (a) my tester tripping SP RCBOs and (B) cost.

Thanks for the NIC opinions .

Deke

 

[batty]

I think you will find that it is, both in the big red red comic and in real life.lets go back to basics now,

disconnect your neutral from your lighting cct(for example),

switch on a light ,

now test between neutral and earth,

and you still want to say the neutral is NOT a live conductor?

what happens on the DNO side of the cut out(where neutral goes to earth potential at the transformer) is of no consequence to you under normal circumstances.

I am sorry steps but you are going to have to state reg numbers that say you cannot use single pole Rcbo's on a TT system in a domestic enviroment. If you look in the on site guide on page 16 It has a picture of a TT system which has a consumer unit full of Rcbo's. On my last assesment and before the job I did for the assement I asked if I could use single pole Rcbo's on a TT system in domestic and was told I could. If this was wrong I would like to Know because I do not like doing incorrect work. I understand what you are saying about a neutral conductor being capable of being 240v but if there is a fault surely the Rcbo will disconnect the live conductor and therefore there will be 0 volts at the neutral then. I cannot understand why a TT system should be any different to any other system of earthing ie if you have a disconnected neutral. You have got me worried with this so I am going to have to read the red book through.

Batty