Garage cu - discrimination

swc · Oct 20, 2012

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

[ElectriciansForums.net] Garage cu - discrimination

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 · Oct 20, 2012

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 · Oct 20, 2012

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

Octopus · Oct 20, 2012

oh dear do we have another troll on our patch?

electric will · Oct 20, 2012

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 · Oct 20, 2012

amateur_25 said:
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 · Oct 20, 2012

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 · Oct 20, 2012

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

Guest55 · Oct 20, 2012

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

swc · Oct 20, 2012

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 · Oct 20, 2012

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 · Oct 20, 2012

Murdoch said:
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 · Oct 20, 2012

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 · Oct 20, 2012

What cu have you bought?

swc · Oct 20, 2012

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

It's a CTI cu

Garage cu - discrimination

swc

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Octopus

electric will

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Octopus

electric will

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Guest55

Richard Burns

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electric will

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Guest55

swc

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Fin170

sparks1973

swc

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Octopus

swc

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