safety device for distribution circuit!

[bluesky]

just visited a house, 3 phase supply, there is a swimming pool in the back garden, the thing worries me is there is only a cut out fuse for the SWA distribution cable, no safety device,cable is about 20m long underground to the shed, what is the best safety device shall i persuade the owner to use, a 100A RCD?

cheers

 

[Deleted member 9648]

Will need a means of isolation and fault/overcurrent protection...whether an RCD is required for the distribution circuit would depend on the earthing system. Much more info needed....such as size of SWA,loading,earthing system,provision of RCD'S at the pool end?

 

[Dunc]

When you say 'a cut out fuse', do you mean the 100A BS1361 (or similar size fuse) that is in the suppliers service head or one of these?

LINK

Probably misunderstanding what you mean at the moment so don't want to comment too much.

 

[IQ Electrical]

just visited a house, 3 phase supply, there is a swimming pool in the back garden, the thing worries me is there is only a cut out fuse for the SWA distribution cable, no safety device,cable is about 20m long underground to the shed, what is the best safety device shall i persuade the owner to use, a 100A RCD?

cheers

I think you'll need to provide a bit more information then that.

Why does the mechanically protected SWA need rcd protection?

Is there rcd protection in the 'shed'?

What is the earthing system?

Is this supplying the pool plant?

Is the 'cut out fuse' the DNO fuse or in effect, a distribution circuit protective device?

 

[mart8235]

RCD is not required on a distribution circuit.
an earthed SWA should be used and this is all you would need.
IF the house is on a tt system then there should/could be a 100mA RCD protecting the whole installation. thats the only requirment.
it doesn't matter if its a BS 60898/3871/88..... thats your protection!! your disconection time is 5sec (if tn system) for distribution so just cjeck yor cable size ect.

 

[IQ Electrical]

RCD is not required on a distribution circuit.
an earthed SWA should be used and this is all you would need.
if the house is on a tt system then there must be a 100mA RCD protecting the whole installation. thats the only requirment.
it doesn't matter if its a BS 60898/3871/88..... thats your protection!! your disconection time is 5sec for distribution so just cjeck yor cable size ect.

Can you point me to that regulation please?

In a '17th edition' consumer unit why would you use a 100mA RCD?

 

[bluesky]

hi, yes there is a 100A switch in a steel box in the main house, 16mm2 distribution cable, 20m away in the shed there is a box which has main switch, 80 RCD and circuits for swimming pool heater, sockets and lights, it is tn-s system, i am thinking what if the SWA get disturbed sometimes such as a spade? without circuite breaker or rcd, then the power wont be off

 

[IQ Electrical]

hi, yes there is a 100A switch in a steel box in the main house, 16mm2 distribution cable, 20m away in the shed there is a box which has main switch, 80 RCD and circuits for swimming pool, sockets and lights, it is tn-s system, i am thinking what if the SWA get disturbed sometimes such as a spade? without circuite breaker or rcd, then the power wont be off

That's why you install mechanically protected cable at a suitable depth with identification tape/tiles.

 

[bluesky]

That's why you install mechanically protected cable at a suitable depth with identification tape/tiles.

hi IQ Electrical, you are right, but i see a lot of garden work going on there, i dont know the depth and how the SWA cable runs, but bit not sure if steel net can protect the cable from sharp spade...

 

[IQ Electrical]

Is this a periodic inspection?

 

[bluesky]

Is this a periodic inspection?

no, just changed a bulb in the shed...

 

[nickblake]

RCD is not required on a distribution circuit.
an earthed SWA should be used and this is all you would need.
if the house is on a tt system then there must be a 100mA RCD protecting the whole installation. thats the only requirment.
it doesn't matter if its a BS 60898/3871/88..... thats your protection!! your disconection time is 5sec for distribution so just cjeck yor cable size ect.

Im with IQ on this one you cant say that the disconnection time is 5 seconds as you dont know the size of the protective device !!! all circuits up to 32 amps have a disconnection time of 0.4 seconds ,above 32 amps is 5 seconds , and again if you have a 17th addition board installed as IQ has stated there is no need for a 100ma RCD

 

[IQ Electrical]

no, just changed a bulb in the shed...

No way!

Change the 'bulb' then and get him to call you if he ever digs through the SWA !!

 

[bluesky]

hi nickblake, do you agree with mart that SWA is enough with distribution cable, as long as there is 17th edition board in the shed?

 

[bluesky]

No way!

Change the 'bulb' then and get him to call you if he ever digs through the SWA !!

just had a quick look of the wiring in the main house, it is really not my responsibility if someone digs through the SWA...

 

[IQ Electrical]

hi nickblake, do you agree with mart that SWA is enough with distribution cable, as long as there is 17th edition board in the shed?

It's not about whether we agree with each other, it's about the requirements of BS7671:2008-do you not have a copy to refer to?

 

[Deleted member 9648]

RCD is not required on a distribution circuit.
an earthed SWA should be used and this is all you would need.
if the house is on a tt system then there must be a 100mA RCD protecting the whole installation. thats the only requirment.
it doesn't matter if its a BS 60898/3871/88..... thats your protection!! your disconection time is 5sec for distribution so just cjeck yor cable size ect.

If it was a TT it would be 1 second for a distribution circuit.....411.3.2.4

 

[bluesky]

It's not about whether we agree with each other, it's about the requirements of BS7671:2008-do you not have a copy to refer to?

i do have one, but cannot find the reference, thinking ask someone who knows it, and poeple could have different understanding, the codes cannot say everything clearly, do you agree?

 

[nickblake]

I wouldnt like to comment unless i have more information regarding the installation , like IQ has said why are you going to all this trouble after just changing the light bulb

 

[bluesky]

I wouldnt like to comment unless i have more information regarding the installation , like IQ has said why are you going to all this trouble after just changing the light bulb

i am newly qualified, i would like to learn everything on site, even some small jobs, i found it is good practice, surely i wont do any suggesstions to the client, but next time i would be prepared when i see it again, i really appreciate that i have got lots of patient help here...

 

[bluesky]

in my opinion, really need a rcd or a circuit breaker at the origin of the distributiion circuit, but at the moment there is only a 100A main swith there...

 

[mart8235]

Can you point me to that regulation please?

In a '17th edition' consumer unit why would you use a 100mA RCD?

are you really asking why i would use a 100mA RCD on a TT system????

 

[bluesky]

are you really asking why i would use a 100mA RCD on a TT system????

hi mart, do you think a 100A main switch is enough at the origin? because the main switch will never trip anyway, so it wont satisfy the 5s disconnection time? at least it is a breaker, sorry cannot find the reference in red book...

 

[mart8235]

Im with IQ on this one you cant say that the disconnection time is 5 seconds as you dont know the size of the protective device !!! all circuits up to 32 amps have a disconnection time of 0.4 seconds ,above 32 amps is 5 seconds , and again if you have a 17th addition board installed as IQ has stated there is no need for a 100ma RCD

All final circuits up to 32A are 0.4s, all circuits above 32A AND DISTRIBUTION CIRCUITS are 5s, therefore all distribution circuits reagrdless of size are 5s......... have look at the table.
only mentioned a 100mA RCD incase it was a TT system, and that would be the only reason you must have an RCD protecting the distribution circuit....... and yes i know the disconnection time is not 5s for TT.

 

[mart8235]

hi mart, do you think a 100A main switch is enough at the origin? because the main switch will never trip anyway, so it wont satisfy the 5s disconnection time? at least it is a breaker, sorry cannot find the reference in red book...

A 100A main switch is a switch and not a circuit breaker. the distribution circuit must be on a circuit breaker/fuse of some sort at the appropriate size to protect the cable. the armourings must be earthed and then no RCD is required.
if the suppliers fuses are 100A or less then diversity is satisfied and there is no reason to consider changing the main switch.

 

[IQ Electrical]

are you really asking why i would use a 100mA RCD on a TT system????

No, I'm not interested in why YOU would use a 100mA RCD, I asked you to point me and anyone that subsequently reads this mess of a thread to the regulation that requires it...

 

[Deleted member 26818]

411.5.3 indicates that an RCD is the prefered protective device in a TT system, and that it can be ommited if a suitably low value of Zs can be permanently and reliably assured.
Not aware of a particular Regulation that states the rating of the RCD, other than that it must meet the requirements of Table 41.5 for the apropriate values of Zs.

 

[IQ Electrical]

411.5.3 indicates that an RCD is the prefered protective device in a TT system, and that it can be ommited if a suitably low value of Zs can be permanently and reliably assured.
Not aware of a particular Regulation that states the rating of the RCD, other than that it must meet the requirements of Table 41.5 for the apropriate values of Zs.

That's all correct Spin but here is my original question:

Can you point me to that regulation please?

In a '17th edition' consumer unit why would you use a 100mA RCD?

 

[mart8235]

That's all correct Spin but here is my original question:

Can you point me to that regulation please?

In a '17th edition' consumer unit why would you use a 100mA RCD?

Please define a '17th Edition' edition consumer unit, as I am unaware the IEE have designed a particular CU.
But no, I will not tell you where regulations are in the book. Its up to you to find them yourself!!! besides, my regs are in work!!!!

 

[Deleted member 26818]

That's all correct Spin but here is my original question:

Can you point me to that regulation please?

In a '17th edition' consumer unit why would you use a 100mA RCD?

You are well aware IQ, that there is no Regulation that requires 100mA RCD protection.
It might be desirable to use a time delayed 100mA RCD to provide discrimination, where there are a mixture of circuits some provided with 30mA RCD protection and others without.

 

[yellowvanman]

There is no mention of a 100mA time delayed RCD in the 17th, except in Appendix 3 (Page 243) to give disconnection times. Its used as a solution for RCD discrimination, whereby the RCD closest to the fault will trip before anything else trips. The time delay gives a 30mA RCD 100ms to trip, if not it trips itself.
A 30mA RCD should trip within 300ms for a 30mA fault (which wouldn't disturb the 100mA RCD), or the 30mA RCD should trip within 150ms for a 60mA (x2) fault, so this could make the 100ms RCD react, but for a 150mA (x5) fault the 30mA RCD should definetly trip within 40ms, if not the 100mA should trip!

RCDs outside Additional protection and the other requirements of a 30mA RCD are used when the potential fault current is not enough to trip an overcurrent device (eg. fuse or MCB)

 

[nickblake]

All final circuits up to 32A are 0.4s, all circuits above 32A AND DISTRIBUTION CIRCUITS are 5s, therefore all distribution circuits reagrdless of size are 5s......... have look at the table.
only mentioned a 100mA RCD incase it was a TT system, and that would be the only reason you must have an RCD protecting the distribution circuit....... and yes i know the disconnection time is not 5s for TT.

Yes i agree i had it in my mind it was a final circuit how ever not all distribution circuits have a 5 second disconnection time , hence the reason why i asked for further information , i was unaware of the CDU in the shed type wether it had an RCD if it had no RCD and supplied Socket outlets it would then be required to disconnect in .4 of a second and require RCD protection

 

[Martin Bradley]

IQ you would not, and i know you know, but people fail to read things.I dont think they have cotton on to what you have said. unless im reading it wrong LOL.

 

[mart8235]

Yes i agree i had it in my mind it was a final circuit how ever not all distribution circuits have a 5 second disconnection time , hence the reason why i asked for further information , i was unaware of the CDU in the shed type wether it had an RCD if it had no RCD and supplied Socket outlets it would then be required to disconnect in .4 of a second and require RCD protection

isn't the Dist board in the garage a separate issue? All those circuits would need to be considered separately? with the most likely answer being RCD main switch or RCBO's in the garage DB.

And guys,
what i was originally saying is that there is no requirement for the distribution circuit to be on an RCD/RCBO provided the cable is adequately protected.
As for suggesting that a TT system should (NOT MUST - big difference between the wording) be on a 100mA RCD then obviously I'm unaware that's the worst thing you can do? only trying to cover the bases!

IQ Electrical what would YOU install on a TT System?
PLEASE STOP SHOUT TYPING AT ME!!!!! I'll tell my mum!!!!

 

[IQ Electrical]

It was the highlighted text in post 6, the word you used was 'must'-check it out.

I just don't want anyone reading the thread to assume that the 100mA front end rcd is the in stone TT setup.

The introduction of BS7671:2008 virtually eliminated the need for the above scenario.

I put '17th edition' consumer unit in inverted comma's to show that the terminology was incorrect but to save describing it's setup.

No more shout typing so don't disturb your mum

 

[Deleted member 26818]

Shouldn't you use the term '(sic)' when indicating something is incorrect?

 

[IQ Electrical]

Shouldn't you use the term '(sic)' when indicating something is incorrect?

You certainly should.

 

[morph]

Im with IQ on this one you cant say that the disconnection time is 5 seconds as you dont know the size of the protective device !!! all circuits up to 32 amps have a disconnection time of 0.4 seconds ,above 32 amps is 5 seconds , and again if you have a 17th addition board installed as IQ has stated there is no need for a 100ma RCD

its 1 s if TT

 

[mart8235]

It was the highlighted text in post 6, the word you used was 'must'-check it out.

I just don't want anyone reading the thread to assume that the 100mA front end rcd is the in stone TT setup.

The introduction of BS7671:2008 virtually eliminated the need for the above scenario.

I put '17th edition' consumer unit in inverted comma's to show that the terminology was incorrect but to save describing it's setup.

No more shout typing so don't disturb your mum

Well..... yup.... it seems I did say 'must'. don't know what that was all about, I know better than that!
I'm only yanking ya chain about the board, but you knew that!
can I amend the post? I'll try now........


..........Yup changed now

 

[morph]

COULD BE 100, 300 rcd depends on ZE