Garage cu - discrimination

[swc]

Hi
Am currently doing my 17th edition. Our teacher gave us a question to split typic domestic loads amongst the 10 way mcbs.
This is what I come up with


However one of my more experieced course mates said "All garage circuits protected by 2 x 30mA RCDs - no discrimination"
I know idscrimination is when the protective device closest to the fault operates. But am confused as to whats wrong with the way I did my consumer unit

 

[Octopus]

Discrimination is not what you put, it is where you have 2 devices of different values in series in the same circuit. In your scheme you have 2 x 30mA and its anyones guess which would trip first.

Hope this helps

 

[electric will]

You have already posted these pictures saying you needed to send them to building control.
so is it for collage or building control???

 

[Octopus]

oh dear do we have another troll on our patch?

 

[electric will]

I don't recognise the user name just the wiring diagrams. Said he'd drawn them up to send to building control before his first registration for NIC

 

[Guest55]

But am confused as to whats wrong with the way I did my consumer unit

nothing wrong with consumer unit layout , just no point in putting a rcd in garage unit.
edit ;
although your circuit split of 3 & 7 over the 2 rcds is a bit unbalanced.

 

[Richard Burns]

Discrimination is the selection of appropriately rated devices so that in the case of a fault the time /current characteristics of the devices are such that the device closest to the fault will trip and clear the fault before the next upstream device will operate.

You have supplied the garage CU from a circuit protected by a 30mA RCD, however you have also put a 30mA RCD in the garage CU so in the case of a fault in the garage the main 30mA RCD in the house CU could trip before the garage one so there is no discrimination between these two devices.
The garage CU would ideally be supplied from a non rcd protected way in the house CU (where the circuit is designed so that RCD protection is not required for the garage supply cable) and the garage RCD will then be the only one to trip if there is a fault in the garage.

 

[electric will]

Found it! Not posted on this forum but tef. User name amateursparky

 

[Guest55]

hes gone anyway , leaving us talking to an empty chair lol.

 

[swc]

Am sorry guys but you can't blame me. All I want is to learn. Can you not understand being the newbie here is difficult. Nobody willing to train you. How heck are supposed to being a spark without training? The college teach you **** all. If I was 16 I wont have this problem! I would do apprenticeship. But am bloody 26 and have nothing to show for it. Am desperate for help. Perhaps am bitting off a bit more than I can chew.

It is for building control part P.

 

[Fin170]

I'd try and get the boiler, garage supply (put on its own 'garage unit'), smoke & burglar alarms on non-rcd ways. Obviously with the 17th edition being what it is that's not always possible but better if you can do it that way (the least amount of circuits down in the event of a fault the better for good discrimination)

 

[sparks1973]

Discrimination is not what you put, it is where you have 2 devices of different values in series in the same circuit. In your scheme you have 2 x 30mA and its anyones guess which would trip first.

Hope this helps

and looking at that diagram...no mention fo wether its a high integrity C/u or not
i mean are there any unprotected ways.......i suppose the fact that it shows 2 spare ways is a clue.....

 

[swc]

Am not sure what the heck have I purchased. Anyway there are two unprotected ways next to the main sw. They're for the smoke detector and alarm.

May I ask why there is a 9 way connector strip for each RCD when there's only 5 mcbs per rcd?

Any chance of cutting them shorter so I can make the unprotected ways N bar longer? If that not possible and I need a new board what do I need and where to buy? I should be able to return the cti one I've got now. Think the shoip owner is quite flexible on that.

 

[Octopus]

What cu have you bought?

 

[swc]

Here's an image of the cu
http://imageshack.us/a/img87/9335/img0002bx.jpg

It's a CTI cu

 

[sparks1973]

What cu have you bought?

the wrong one....lol

 

[Octopus]

hi integrity

 

[swc]

hi integrity

OK my bad I brought the wrong one. What the heck is a high integrity consumer unit? whats the difference between that and one I have brought?

You what's even more funny, the first one I brought was a hi integrity unit if am not mistaken can check. I can't return it since they'll take 15% restocking. I was going to use it at another house we own. I know for a fact it's not a 17th edition board because we brought the property before 2008! It only has one RCD as well.

 

[sparks1973]

well what it is..

generally we use whats known as a `split load board`....2 giving additional/fault protection to 2 sides....they`l be `split`...typically a 5 n 5 setup....
a high integrity board has this....plus 2 additional ways for circuits not given aditional/fault protection....now...
you need one of these regardless of the earthing arrangements...(by the way what are these please)
then (depending on earthing arrangements)....you use a cable (with earthed mettallic covering)..:such as an SWA...or MICC...to feed the garage...now,
depending on the earthing arrangements you have at the head.....will depend on wether you require a 100mA s type RCD..(hence the `high integrity` aspect) of the board ( 3 neutral bars)

 

[Deleted member 26818]

At present, there are 2 unprotected ways.
You could extend the existing bus bar, but then you would not have enough spare ways in the neutral bar.
Another option would be to place the smokes on a lighting circuit and use the space freed up for the garage.

 

[oldAL]

actually, Ive looked at his original thread on that other place and all the optionjs have already been offered from what I can see,
do people ever read what is replied to them?
on any forum?

 

[Fin170]

Here's an image of the cu
http://imageshack.us/a/img87/9335/img0002bx.jpg

It's a CTI cu

Personally I'd put that one back in the skip :lol: Go down to B&Q and pick up the BG one for £50 that you can set up properly


Edit: Bugger didn't see the 2nd page posts

 

[not clever]

The cu you have is a high integrity board, the first two ways not protected by an RCD are the integrity ways & are supposed to be for non rcd protected circuits or the use of rcbo's. Why chuck it? if it meets all the BS requirements use it .

As for your set up I would go with the Smokes & security alarm on one circuit by using one of the high integrity ways & an rcbo.
The second high integrity way I would use for the garage & make the circuit such that an mcb will suffice as the protective device. This will get away from your set up having 2x 30mA rcbo's in series.
The garage cu keep the main switch as a 30mA rcd.

All other circuits spread across both rcd protected ways.
I take it both socket circuits will be ring final? as you have one as "socket circuit" if it is intended to be a radial, then you'll need to use 4mm T/E or drop the ocpd to 20A if using 2.5mm as you have listed.

 

[PC Electrics]

Some good stuff from nc there. I'd only add that I would put the downstairs lights on an rcbo along with th ealarm and smokes. That way if the alarm/smokes loose power for any reason you will know about it 'cos your lights are out. Otherwise a loss of power to these critical items could go un-noticed.

As to some of the other comments, well Amateur, you have been inconsistant with your whys and wherefores and your description of your background. On forums, it is much prefered if people with questions are totally honest about themselves and why they are asking. Not being so damages your reputation and makes it difficult to taylor answers to the level required.

 

[swc]

the rcbo has a neutral wire where can i put that when i only have two spare ways in neutral bar which are for the two unprotected ways.

 

[PC Electrics]

the rcbo has a neutral wire where can i put that when i only have two spare ways in neutral bar which are for the two unprotected ways.

Uh oh.......

I can see a flaming coming on.

I'm out!

 

[delr]

the rcbo has a neutral wire where can i put that when i only have two spare ways in neutral bar which are for the two unprotected ways.

Hi folks
first post on the forum - enjoyed reading many of the topics posted during the past few months - but some of the questions asked scare the **** out of me:sad_smile:

 

[Octopus]

Hi folks
first post on the forum - enjoyed reading many of the topics posted during the past few months - but some of the questions asked scare the **** out of me:sad_smile:

And that's from some of the "sparkies", just wait until you see some of the DIYers questions!!

 

[Brightspark2]

The college teach you **** all.

Thats rubbish, not a 5week course but the right course at collage will give you the right training. You dont need to be 16 to do the course theres adult learning on the same courses (NVQ, C&G and then complete your AM2). It will take the same amount of time 4 years and you dont need to go into collage each week as they trust you to do work at home unlike the apprentices. It would also include teaching about discrimination.

You may be biting off more than you can chew, what qualifications and training do you have?

If you want to learn which it seems you do then above is the way to do it, you wont learn by guessing your way through the job. Dont forget the dangers electricity can create.

Im not being funny with you here, just trying to give you advice.

Never heard of CTI either...

 

[not clever]

I'm with PC ................. bye

 

[Richard Burns]

the rcbo has a neutral wire where can i put that when i only have two spare ways in neutral bar which are for the two unprotected ways.

Okay what you need to do is to be able to think about what you are going to do and how this will affect the circuit you are wiring up.
As you are in training (or possibly not) you still have some way to go with the understanding of circuits and how they are connected and how they will interact.

If you are using an RCBO then the neutral wire for the circuit goes into the RCBO and the neutral lead from the RCBO goes into the neutral bar, so you will not have a problem with too many neutral cables, even so, if you are thinking about it, you will see that you could put two neutral cables in to the same neutral termination.

 

[Deleted member 26818]

the rcbo has a neutral wire where can i put that when i only have two spare ways in neutral bar which are for the two unprotected ways.

When you use an RCBO, the circuit neutral is connected to the RCBO, rather than the neutral bar.
The neutral lead on the RCBO then goes to the neutral bar instead.
An RCD/RCBO detects any difference in current between the Line and Neutral.
As such the neutral has to be connected to an RCD/RCBO in order for the device to detect the difference.