Rcd- mcb

[davidcaulfield]

Hi guys in installing a 16mm submain with a 10mm earth and I'm going to install a 80amp time delay RCD with a 63amp MCB to protect it. Some one said I could just use a 63amp RCD to protect it? Never done this before and wouldn't think you could.

Also should I just use a 63amp RCD with a 63amp MCB

 

[trev]

Nowhere near enough info.

 

[danzor]

What's the sub main? T+E or SWA?

If T+E put a time delay on it, then 30mA at the new board

 

[Darkwood]

Are you suggesting having no overload protection on your sub-main supply?? More info please.

 

[telectrix]

does it need RCD protection?

 

[Guitarist]

Agree, not enough info. Very unlikely a time-delayed RCD will be of any use to you anyway.

 

[DPG]

Are you thinking that the 63A RCD will trip on overload of >63A ?? Daz

 

[telectrix]

it's a common misconception that RCDs and Main switches ( e.g. BS 60947-3 ) will trip if the rated current is exceeded. keeep telling guys that the A rating of a switch/RCD is just the load it's designed to carry.

 

[DPG]

A spark carrying out work for people would know this though?? Daz

 

[Jay Sparks]

A spark carrying out work for people would know this though?? Daz

You'd be suprised!!!!

 

[telectrix]

you'd think so, but how many sparks don't understand the principles of bonding? it's not done " in case water/gas pipes become live", but it's to ensure that they rise to an equipotential with the fault so that the p.d. is as near to zero as possible. not easy to explain.

 

[davidcaulfield]

Sorry guys for the quick writing

I am going to protect the 16mm T&E submain with a time delay RCD and a 63amp MCB. The guy I was working for buys all the bits and said I should jut use a 63amp RCD. I have never heard this and didn't think they protected against overload but don't want to argue with him if there a chance he's right (which I doubt) just think I would ask on here if all these years I have been doing it wrong

The time delay RCD will be feeding a 2 bed granny flat which will have a 17th edition board.

 

[davidcaulfield]

The main question I was asking is if I'm using a 63amp MCB, what rating time delay RCD would you normally use... 63amp , 80amp , 100amp??

They only had 80amp in stock so I said this will do?

 

[DPG]

It's the max current the RCD can work at, not what it trips at. Daz

 

[Darkwood]

You seem to be in confusion because someone has contradicted what you know.... RCD's dont protect against overload and the Amp rating is the max that the unit can safely carry like a 15amp connector will carry up to 15amps so in essence of this fitting a higher rated will be fine as would using 30amp connectors to replace 15amps even though the load would not exceed 15amps.

We questioned why you think you need a time delayed rcd in the first place?
We lack alot of info'

 

[davidcaulfield]

The time delay RCD protect the T&E submain when running surface in the garage, underground , then up a dot and dab wall to the consumer unit.

I no it doesn't need RCD protection as its a distribution circuit but surely prevention is better than cure?

I just thought if some one ever damaged it or drills it then the RCD could trip out in 0.04 second? Potentially saving a life?

Or am I getting it wrong as I've never used one before???

 

[Darkwood]

Regardless of it been a sub-mains supply it would still be subject to the same regulations that all other circuits are regarding 30mA protection unless its was say an SWA .... so i query the fact to it having time delay surely fitting a standard front end 30mA rcd is the way to go and all the board is then protected.... if you didn't want the tripping to drop out the supply to numerous circuits you should have wired it in swa then rcbo'd the circuits.

If you dont have freezers etc in the garage this isn't really an issue and front end protection is fine.

How is the cable protected underground is it fully enclosed throughout in conduit or you just run it through plastic ducting? Also what depth?

 

[GLENNSPARK]

it's a common misconception that RCDs and Main switches ( e.g. BS 60947-3 ) will trip if the rated current is exceeded. keeep telling guys that the A rating of a switch/RCD is just the load it's designed to carry.

yep....and it would be funny if it wasn`t just so poor....

 

[DPG]

I wonder how many installations out there have circuits protected by an RCD only, ie. where the spark is thinking that the device is giving overload protection as well as the RCD function. There could be a lot of dangerous situations arising from this. Daz

 

[GLENNSPARK]

to the O/P:
the lads need to know the earthing arrangements for a start...

what type of cable you intent to use for this sub...

whether or not theres any extranious bottom end...

what the max demand is going to be...

 

[Guitarist]

As dark says, you can't just discard RCD protection because it's a distribution circuit! If it's T&E you will need 30mA, not 100mA TD.

 

[GLENNSPARK]

and we still dont know what the earthing arrangements are.....

 

[Colin Jenkins]

Hello David.

The RCD is there for the protection of personnel who may inadvertently come into contact with live a.c. mains. This will not happen on a sub-distribution cable, so no RCD is required for this. The cable itself requires protection from short circuit conditions, and this is what an MCB does. A correctly-rated MCB will be required at the supply end. If personnel protection is deemed necessary for the final sub circuits, this should be provided at the remote end of the supply cable. Nuisance trips become an issue if this is installed at the supply end. If anyone sticks a drill or similar into the cable, the MCB will oprerate and disconnect it.

Regards,

Colin Jenkins.

 

[peanutty]

Hello David.

The RCD is there for the protection of personnel who may inadvertently come into contact with live a.c. mains. This will not happen on a sub-distribution cable, so no RCD is required for this. The cable itself requires protection from short circuit conditions, and this is what an MCB does. A correctly-rated MCB will be required at the supply end. If personnel protection is deemed necessary for the final sub circuits, this should be provided at the remote end of the supply cable. Nuisance trips become an issue if this is installed at the supply end. If anyone sticks a drill or similar into the cable, the MCB will oprerate and disconnect it.

Regards,

Colin Jenkins.

still not right!!
pretty sure if no mechanic protection and not surface mounted etc, basically it will need a 30mA RCD and correct rating MCB (or just correct rcbo if no double pole isolation is needed).even if its a sub main feed it can still be damaged.

what type earthing? can it be used?

 

[Geoffsd]

The RCD is there for the protection of personnel who may inadvertently come into contact with live a.c. mains. This will not happen on a sub-distribution cable, so no RCD is required for this.

That's not correct.
The cables of a sub-main are subject to the same requirements as any other circuit.
That is, if new and concealed less than 50mm. from a surface and not mechanically protected then an RCD is required.

Nuisance trips become an issue if this is installed at the supply end.

Nevertheless an RCD is required if installed as above.

If anyone sticks a drill or similar into the cable, the MCB will oprerate and disconnect it.

Not if you only strike one of the conductors.
That is why an RCD is required.

 

[D Skelton]

A spark carrying out work for people would know this though?? Daz

When you've been told something wrong by a number of people you start to doubt your own knowledge. I was taught in college by more than one tutor that an RCD would trip upon the rated load being exceeded. I was also told this by one of my supervisors at the time. I was ridiculed for questioning my tutors knowledge so I accepted it. Years later on here I was ridiculed for not knowing that this indeed wasn't the case.

Dammed if you do and dammed if you don't!

 

[GLENNSPARK]

When you've been told something wrong by a number of people you start to doubt your own knowledge. I was taught in college by more than one tutor that an RCD would trip upon the rated load being exceeded. I was also told this by one of my supervisors at the time. I was ridiculed for questioning my tutors knowledge so I accepted it. Years later on here I was ridiculed for not knowing that this indeed wasn't the case.

Dammed if you do and dammed if you don't!

well surely that makes these `tutors` not fit for purpose then doesn`t it Damian...

 

[D Skelton]

I'd say 90% of tutors these days are not fit for purpose! Most of them have never spent a day in their life on site. They qualify and then go straight into teaching. It is very easy for a younger guy who wants a way into the industry to put muppets like these on a pedestal. Its only when they have a bit more experience and have done their homework themselves that they realise what a sham they have bought into!

 

[GLENNSPARK]

I'd say 90% of tutors these days are not fit for purpose! Most of them have never spent a day in their life on site. They qualify and then go straight into teaching. It is very easy for a younger guy who wants a way into the industry to put muppets like these on a pedestal. Its only when they have a bit more experience and have done their homework themselves that they realise what a sham they have bought into!

the lecturers at the college i was at were all ex pit guys....all of em had come through the NCB....
although one of em didn`t believe in applying diversity to anything...??????...lol..

 

[Darkwood]

Hello David.

The RCD is there for the protection of personnel who may inadvertently come into contact with live a.c. mains. This will not happen on a sub-distribution cable, so no RCD is required for this. The cable itself requires protection from short circuit conditions, and this is what an MCB does. A correctly-rated MCB will be required at the supply end. If personnel protection is deemed necessary for the final sub circuits, this should be provided at the remote end of the supply cable. Nuisance trips become an issue if this is installed at the supply end. If anyone sticks a drill or similar into the cable, the MCB will oprerate and disconnect it.

Regards,

Colin Jenkins.

You contradict everything i put here so i call on you to express what regulations allow a submain supply to not require rcd protection as a rule then i show you several regulations that say you are wrong regarding what limited info we have given..... please don't give advice if you don't know what you are talking about and if you think you are correct you need to either shoot your tutor or do a full 3-4yr training course.

PS what would happen if someone drill the dot and dab wall and caught the cable with the live only touching the drill.... the mcb wouldn't trip and the drill may become live....... your advice could be dangerous!

Reading your profile im shocked to find your a consultant yet you just fluffed on what are basic regulations that have been in practice for yrs.... this is not an industrial set-up where the use of swa or all surface mount systems may have been installed this is a supply to a submains in a domestic situe and like every T&E cable it is subject to the same regulations as all the final circuit cables....

Also as highlighted by Glensparks its crucial we know the nature of the supply as well as the construct of the garage to ensure the earthing arrangement complies as it may be a case of he's exported the the earth and left a dangerous senerio.

Shocked i am!! (pun not intended)

Dont get me wrong your knowledge and experience in Electrical Engineering may be valuable but you seem to still hovering on 15th to early 16th edition regarding the IEE regulations but if you think you are correct i dread to think what as a consultant you have got wrong!

 

[Guitarist]

Hello David.

The RCD is there for the protection of personnel who may inadvertently come into contact with live a.c. mains. This will not happen on a sub-distribution cable, so no RCD is required for this. The cable itself requires protection from short circuit conditions, and this is what an MCB does. A correctly-rated MCB will be required at the supply end. If personnel protection is deemed necessary for the final sub circuits, this should be provided at the remote end of the supply cable. Nuisance trips become an issue if this is installed at the supply end. If anyone sticks a drill or similar into the cable, the MCB will oprerate and disconnect it.

Regards,

Colin Jenkins.

I am also shocked by this advice. (No pun intended either)
Poor advice is more dangerous than none at all.

 

[Flanders]

Hello David.

The RCD is there for the protection of personnel who may inadvertently come into contact with live a.c. mains. This will not happen on a sub-distribution cable, so no RCD is required for this. The cable itself requires protection from short circuit conditions, and this is what an MCB does. A correctly-rated MCB will be required at the supply end. If personnel protection is deemed necessary for the final sub circuits, this should be provided at the remote end of the supply cable. Nuisance trips become an issue if this is installed at the supply end. If anyone sticks a drill or similar into the cable, the MCB will oprerate and disconnect it.

Regards,

Colin Jenkins.

This was why RCBOS /RCDs have to installed on all circuits that are not mechanically protected or deeper than 50mm within fabric of building now because apparently some MPs wife put up a metal spice rack with the fixing screw hitting the live conductor and making the spice rack live. The screw missed the earth so did not trip the MCB

 

[Colin Jenkins]

Hello David.

With apologies, please ignore my earlier response - it's total rubbish.

As stated above, if the twe cable is hidden within the building fabric at less than 50mm depth and it is not mechabnically protected, then an RCD will be required at the supply end for personnel protection. Please note that a conventional 30mA/40mSec max. RCD should be selected for this; a time-delayed unit will not meet the requirements. The current carrying capacity of the RCD should match or exceed the rating of the selected mcb.

63 amps for the mcb sounds about right, but you will need to do the cable selection calculations accounting for Ze, the installation method and the total circuit length to confirm this.

Regards,

Colin Jenkins.

 

[Darkwood]

Hat off to you Colin for holding your hands up and expressing your original post was 'total rubbish', many members on here would just go into hiding when they mess up..... having said that im a little worried that amongst all the members that made such a basic error it was a consultant of many yrs experience as i struggle to see this error was made due to misinterpretation of the OP as he was clear regarding the run of the cable....

We all make boo boo's though even me and at least you took this one on the chin! :smartass2:

 

[Colin Jenkins]

Hello Darkwood.

Thank you for your note and your thoughts. This forum is for all of us to make our best efforts to assist others; if an unintentional goof-up gets in the community promptly and politely corrects it, as has happened here. My first response was framed in industrial-think (where I spend most of my time). A "steel conduit and SWA" world, although the OP clearly stated he was installing TWE!

Once again, I apologise for the hassle my reply caused and have gone to the Dunce's Corner for a while.

Regards,

Colin Jenkins.

 

[Guitarist]

Hello Darkwood.

Thank you for your note and your thoughts. This forum is for all of us to make our best efforts to assist others; if an unintentional goof-up gets in the community promptly and politely corrects it, as has happened here. My first response was framed in industrial-think (where I spend most of my time). A "steel conduit and SWA" world, although the OP clearly stated he was installing TWE!

Once again, I apologise for the hassle my reply caused and have gone to the Dunce's Corner for a while.

Regards,

Colin Jenkins.

Nice to hear Colin. Compared to "other" electrical forums, this one usually gives out excellent advice, and I'm glad to hear that you're big-enough to hold your hands up when you realise you goofed up.

 

[davidcaulfield]

Hi guys the T&E was installed before I quoted the job so swa is not an option.

It does need RCD protection because its being installed in a dot and dab wall.

I am fitting a dual RCD board to 17th edition

But want to protect the submain with a time delay RCD to avoid nuisance tripping?

All circuit will be protected by 30ma RCD but the submain will be on a time delay RCD to avoid conflict between the two Rcds.

I have always installed a 30ma RCD on the submains but when a fault happens writhing the property it can take out the RCD front end which means a long walk to re set?

Hope that makes sense

The earthing system is tncs (not sure why that matters to this)

 

[D Skelton]

If the submain is T+E and installed less than 50mm in a wall then it will need additional protection by way of a 30mA RCD. A Type-S is not suitable for the provision of additional protection.

 

[davidcaulfield]

So you have to use 2 30ma Rcds inline with each other?

 

[D Skelton]

Ideally, the submain would be SWA so you wouldn't have to. Two RCDs inline is usless because you can't discriminate. Is there any way for you to be able to protect the T+E with an earthed metallic plate?

 

[Darkwood]

Deleted OP whipped my butt then i realised i mixed this thread up with another Oopps!:bucktooth:

 

[davidcaulfield]

Yea I could get round that.

It's just my old boss use to use time delay RCDs all the time to stop nuisance tripping

When he did a periodic report he would list two 30ma RCDs inline.

On an existing insulation with cables installed is there Any way of protecting the consumer unit and submain without discrimination?

Where should a time delay RCD be installed then?

 

[D Skelton]

Time delayed RCDs can be used to protect submains to achieve a specific disconnection time allowing for discrimination with other RCDs further on in the circuit. They cannot be used for additional protection, only 30mA RCDs and supplementary bonding can be used for additional protection.

If you can protect the T+E with an earthed metallic plate at least 5mm thick then do that, if you can't, you either put up with having the whole submain on one RCD or rip out the T+E and replace with SWA or similar.

 

[davidcaulfield]

Dark wood i find you a bit rude and insulting.

I have not installed the submain

The will be not be exporting the earth as it it's a granny flat connected to the existing house. Basically a big extension?

My old boss used a time delay RCD and I was just after more information on this method as I've never done it myself. (Obviously he was not doing it correctly)

I came on here for a bit of friendly advice that's all. In future I think I will not ask questions to improve my knowledge and try to keep learning just incase I get this sort of response

Thanks for putting me off this site

 

[Darkwood]

Apologies David i actually mixed your query up with another thread!


I suggest you throw away your teachings from your old boss as it sounds like his --- and elbow must have looked very similar to himself, the supply cable while it stays as is requires rcd protection 30mA - No time delay

Take the board out and fit a standard board mainswitch with mcb set up's or replace the submain supply with a mechanically protected cable like swa.

 

[davidcaulfield]

Take the board out and fit a standard board mainswitch with mcb set up's or replace the submain supply with a mechanically protected cable like swa.

Does this not mean there's no dual RCD protection?

Every time the RCD trips they have to go to the garage to re set it as opposed to going to the local consumer unit inside the granny flat?

 

[D Skelton]

Yep, that is the nature of your predicament.

 

[Darkwood]

Can i just make sure we have this set-up correct...

Main house with built on garage- mains supply comes up in garage and you are sub-maining to a built on granny flat extension, effectively structurally all one building then?
No gaps between the buildings?

Sorry but re-reading the thread you gave us a different impression when you mention garage and submaining to granny flat thus exporting earth would be needed to be assessed.

 

[davidcaulfield]

Hi the granny flat is directly connected to the main house. The garage is connected to the main house Aswell. Sorry for any confusion

Reading other threads they say exporting the earth doesn't really make a diffrence whether is Tns or tncs. I do need to swat up on this.

I understand it if its an outhouse less than 2metres away with no extraneous conductive parts it doesn't need a tt system? Such as an outhouse with a socket and light

 

[davidcaulfield]

D Skelton thanks for you advice and knowledge.

I am looking to do a job of 30 bed sits

I don't want to use swa ( not enough room to run them all through the joists)

I will have to use 2 30ma rcds inline again?