MCB discrimination?

[PhilipKS]

Hi all,
I understand about RCD discrimination, but should the same theory be applied to MCBs / Main switches, or is it just ‘convenience’ to have the mcb trip nearest to the circuit protected?)

I have sub main with a 25A 30mA RCD main switch, coming off a 16A MCB on the main board (protected by a 100mA RCD). I want to add a sub-sub main, but while RCD discrimination is okay - there is currently no discrimination on the MCBs. What rating should I go for to protect the sub-sub main.

Can't go above 16A as I'm into unacceptable voltage drop range.

thanks
Philip

 

[RF Lighting]

Hi all,
I understand about RCD discrimination,

Are you sure?

but should the same theory be applied to MCBs /

Yes

Main switches,

How would that work?

or is it just ‘convenience’ to have the mcb trip nearest to the circuit protected?)

No

I have sub main with a 25A 30mA RCD main switch

Why the RCD main switch?

coming off a 16A MCB on the main board

That's small for a submains

(protected by a 100mA RCD)

Is that a time delay type?

I want to add a sub-sub main

A submains then

but while RCD discrimination is okay

Is it?

there is currently no discrimination on the MCBs.

Which MCBs? You haven't installed the new circuit yet.

What rating should I go for to protect the sub-sub main.

Depends on lots of factors such as the intended load for the submain, cable sizes etc.

Can't go above 16A as I'm into unacceptable voltage drop range.

I dont see how MCB rating affects this. That is caused by undersized conductor CSA.

thanks

It's ok.

Philip

RF Lighting

 

[JohnD]

what is your final circuit for, and what is its design current?
(don't say "growing lamps")

tell me how you think the two RCDs will discriminate.

 

[Taylortwocities]

Discrimination between fuses/cbs is not an easy science. You'll need to know and understand the time/current characteristics of fuses and cbs.

As a rule of thumb, if the ratio between the fuse ratings is 1 : 1.6 then discrimination is usually achieved.

If you want some bedtime reading look here: http://www.memonline.com/cb2.html

 

[securespark]

Thanks, TTC!

I'll print that off, take it to bed & maybe that'll cure my insomnia tonight!!

 

[PhilipKS]

Thanks all.

RF Lighting

I understand about RCD discrimination,
Are you sure?

OK! ..read "I understand the need for and something of the principles about RCD discrimination"

Why the RCD main switch?

Ouside power - The submain serves "professionally installed" garden lighting and a single double socket in a garden building (more detail below)

Is that a time delay type?

Yep - 100mA S-type legrand as the main switch on the main consumer unit

John D - Hence my (simplistic) thinking that that provides the time discrimination, when combined with the 30mA RCD on the sub-main consumer unit.

Which MCBs? You haven't installed the new circuit yet

Current (no pun intended) arrangement is this, I'm just trying to evaluate if what is installed is correct / safe and then look at options (DIY or Pro) to extend the radial circuit serving the garden building final socket to add a couple of other sockets and some internal & external lighting (part p...)

Untitled

• PhilipKS
• 3 May 2008
• 1

TTC - thanks, I'm learning! Found another explanation as well

http://www.voltimum.co.uk/news/6114//Guide-to-discrimination.html?fullsize=yes

Anyone after the full background
//www.diynot.com/forums/viewtopic.php?t=132879
Think I killed the good folk here with too long an original post

Cheers

 

[securespark]

I was just drifting off, then you woke me up!

 

[PhilipKS]

sorry!!

couldn't sleep so thought that surfing the web for stuff on RCD discrimination might send me to sleep...amazingly it didn't!

 

[securespark]

Don't apologise! I'm up late/early myself & can't get to sleep...