Rcd- mcb

[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?