Rcd- mcb

[Darkwood]

This is the info we lacked when you progressed through your thread and we have to assume by what you post ....hence a few of us brought up the exporting earth ...if it was a garage several meters away and of a exposed steel contruct then this would be a possible situation for TT-ing the garage. We now know this is all irrelevant as its not the case... but i think you are now clear with where your issues lie regarding needing to protect the submains with both overload and 30mA rcd protection.

Yes your in a corner if the rcd trips then to the garage it is.... only way around it is to mechanically protect the existing sub supply or replace the existing with SWA.... you have your options now put your argument forward for replacing the sub mains cable.

You could just have the set up as is with 63amp mcb and 30mA submain protection on the 17th edition board and yes poor design and not professional to leave as such but if not your design just argue your corner and do within reason what they ask.

 

[Amp David]

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.

What a load of balls. Never heard so much rubbish in a single post

 

[Darkwood]

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?

No you won't if this is the case the design is wrong and you should change it, if you have other trades on site you have a site meeting you express the need to run x amount of swa cables maybe a riser or ducting can be formed by the builders as a cable run.

Ive just done 20 block of flats all flats have a swa DB supply the cable runs were discussed before pricing and the layout of the room plans had to be altered to allow for my supplies.

Dont box yourself into a corner you can't get out off ..... your the one they will ring when the front end rcd trips especially if the tenants don't have access to say the landlords meter room.

 

[Darkwood]

What a load of balls. Never heard so much rubbish in a single post

Read on further he's already taken the firing squad and admitted the dunce award.

 

[Darkwood]

Lastly a point no one has brought up ....because the submain is a twin and earth cable the earth would be required to be the same size as the live conductor as it supplies earthing for multiple circuits .... if you haven't run additional earthing you will have to show the earth complies to 543.1.2 (i) ---i.e. with use of the adiabatic equation ....

the above is something to remember if you doe run the proposed flats in T&E..

 

[GLENNSPARK]

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

this is not competance david....
its babyish...

 

[Guitarist]

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

Both darkwood and D Skelton have spent a lot of time trying to help you with something which, as you have accepted this job, you should really already know. Be grateful.

 

[Darkwood]

Both darkwood and D Skelton have spent a lot of time trying to help you with something which, as you have accepted this job, you should really already know. Be grateful.

In his defence this comment was justified as i wrote a now deleted post where i mixed my threads up by accident and replied in essence of another thread.

 

[Aindowsimon]

Wot is the issue with a 30mA type s rcd ? All I know is that supplementary protection is desired by the sub-main and by a 30mA rcd is there a reg on that the type s rcd ar not suitable for additional protection ? . Why don't you surface wire the t+e ? I understand it isn't aesthetically pleasing , neither is it convenient to lose all you're circuits to a lamp failure

 

[D Skelton]

Good luck trying to find a 30mA Type-S!

 

[Darkwood]

Lets clear this up a bit ......

Rcd's in parallel - general rules.

Putting RCD's in series would give discrimination issues
As the circuit is a distribution circuit reg' 411.3.2.3 allows a disconnection time of 5s
As the dist' circuit is exceeding 32A reg 411.3.2.2 means it dosn't have to trip in 0.4s
Given the above you CAN have 2 rcd's in series if the distribution circuit rcd is time delayed as a time delayed rcd will trip in <5s.

This still wont achieve discrimination though as a N-E fault would still trip both rcd's so to prevent this you can ensure all downstream devices requiring rcd protection after the distribution circuit are protected by a rcd that breaks BOTH live and neutral. reg 531.2.9

So yes you can have 2 in series as above and comply but you need to confirm that the 17th edition board has double pole rcd's as most do if you are fitting a board with rcbos anywhere in it even fitting rcbo's within the unprotected ways of the 17th edition board then you will have issues as these are not double pole and you wont achieve discrimination.

In the design stage just having an swa to start with saves alot of time and can be cost effective believe it or not after all the supplementary earthing with the sub-main or time wasted trying to ensure the earthing in the t&e is sufficient.

Additional edit ....as had breakdown and didnt have time to look it up...

Your set-up with cable <50mm and no mechanical protection!

Now for the technical bit ........ Reg 522.6.7 species that if all points of reg 522.6.6 are not met in this case a t&e less than 50mm below surface without mech' protection then RCD protection is required-- so far straight forward then it refers to the characteristics of the rcd must comply with reg 415.1.1. which specifies that the rcd must not exceed 30mA and an operating time not exceeding 40ms at a residual current x5.

Thus we now refer to appendix 3.... now this is where as pointed out earlier you confuse us... time delay rcd's start from rating 100mA but looking at the table and assuming you can purchase a 30mA rated S-type it shouldn't effect the trip times against other ratings of S-Type, if you look in the X5 column the tripping characteristics clearly show S-types trip between 40 and 150ms which would mean your install would contravene regulation 415.1.1 because you are using rcd to provide additional protection so it must trip within 40ms at x5 which cannot be done with a time delayed rcd so a standard rcd must be fitted and then we are back to square 1 as you dont have discrimination of rcd's.

Solution ...Swa or othe mechanically protected set-up.

Its ok that your old boss was laying down the rules to you but he also needs to learn his job he's used a set of regulations to show compliance but ignored others that have priority on the situe..... hopefully after all my input here we cn lay this to rest that you can't have in your design a time delay rcd covering the sub-main because it is required for additional protection so must be a standard rcd 30mA with no delay.

 

[telectrix]

apologies if this has been suggested before. i've not read all the posts. from what i gather, it's the T/E installed in the dot&dab wall that's the problem. ( 522.6.101). this could be overcome by enclosing the T/E in earthed steel conduit for the relevant part of the cable , thus negating the need for a RCD upstream.

 

[Darkwood]

POST 61.... this is how i interpret the regulations in the OP's case regarding having a time delayed rcd on the sub mains to try achieve discrimination .... feel free to pick at it as you will as we all know the reg's aint always straight forward and can be interpreted in differing ways...