My last 'non Amd 3' CU Install.

[Simonslimline]

Will have a flick through my books and see if I can dig it out

As an example - The first company i worked for had an office in a small industrial unit, TT installation with an upfront 100ma RCD, SWA sub-main from switch room to unit. A new consumer unit was put in the office we used and had 30ma RCD's. Both RCD's would operate in the event of a fault, Resulting in going to the switch room to reset.

 

[hightower]

Hmmmm, just gone to the page about selectivity in the book I'm currently reading and you guys are right - it says it should be s type in this case. In going to continue flicking through my books as in not sure what the hell I've read giving me this idea

 

[Leesparkykent]

Why type s? I thought 100ma would offer enough discrimination from a 30ma? Read somewhere 3 times as much, so for discrimination 90ma or higher should act independently without the need for a time delay

An S type RCD is also known as a time delayed RCD. If a 100mA RCD is installed upstream of a 30mA RCD, there will be no certainty as to which RCD will trip first. Cascading of General Type RCDs is therefore not recommended. Discrimination can only be provided by ensuring that the upstream RCD has a trip time and trip level envelope which is completely outside that of the downstream RCD. This can only be assured by using an S Type RCD upstream and General types downstream.

 

[hightower]

Last paragraph is where I'd read it, doesn't mention s type until much later hence my confusion

 

[shanky887614]

what book is that hightower?

 

[Simonslimline]

Last paragraph is where I'd read it, doesn't mention s type until much later hence my confusion

Good that you showed us where you got the info from.

In reality though, it does not really work like that.

 

[hightower]

what book is that hightower?

Electricians guide by John Whitfield

 

[shanky887614]

Electricians guide by John Whitfield

p52 anyway that section is referring to mcb etc and that line is talking about discrimination between fuses/mcb

have a look at the table below (i thought i recognised the book ive got the 3rd edition)

 

[hightower]

Good that you showed us where you got the info from.

In reality though, it does not really work like that.

Yeah it does later go on to say a 100ma rcd will likely go as well as a 30ma so type s is brought in to the equation, just about 120 pages on so I wasn't there yet haha

 

[hightower]

p52 anyway that section is referring to mcb etc and that line is talking about discrimination between fuses/mcb

have a look at the table below (i thought i recognised the book ive got the 3rd edition)

Yep can see mcb, strange place to add an excerpt about rcd I think as all it did was throw me off

 

[shanky887614]

Yep can see mcb, strange place to add an excerpt about rcd I think as all it did was throw me off

i havent read all of it but some bits might confuse people.

ive got the older version but its more or less the same word for word in that paragraph

 

[Midwest]

as previously asked is this a tt system? Theres no dno earth and you havent got a rcd main switch at origin.?

Do ya think Dave OCD is visiting relatives today, or just board with the whole thing now?

 

[bigspark17]

Why type s? I thought 100ma would offer enough discrimination from a 30ma? Read somewhere 3 times as much, so for discrimination 90ma or higher should act independently without the need for a time delay

Without knowing that this is not how you discriminate between rcd id like to know how you think youre in a position to offer advice on tt earthing systems?

 

[davesparks]

Without knowing that this is not how you discriminate between rcd id like to know how you think youre in a position to offer advice on tt earthing systems?

Where has he offered advice on TT earthing?

 

[hightower]

Without knowing that this is not how you discriminate between rcd id like to know how you think youre in a position to offer advice on tt earthing systems?

Mate, see that thing under my name that says trainee? Cut me some slack OK? I wasn't saying I was right - merely stating my understanding which other members have kindly corrected me on. Not everything has to turn in to an argument, but I'm not sure you grasp that after having a glance through your posting history. Nearly everyone on here is extremely helpful, especially for people like me. I wish the same could be said about everyone, but comments like yours do the square root of jack all to add to this awesome outlet.

 

[Simonslimline]

Without knowing that this is not how you discriminate between rcd id like to know how you think youre in a position to offer advice on tt earthing systems?

Would still be great to admire a bit of your work as well, if posting a pic is not too much trouble.

 

[GMES]

Without knowing that this is not how you discriminate between rcd id like to know how you think youre in a position to offer advice on tt earthing systems?

Hightower is a trainee which is clearly stated below his username and therefore is merely asking questions to better his learning.

**EDIT*** Hightower beat me to it but my point still stands!!

 

[bigspark17]

how? it is relatively easy in a lot of places to get a really low ra so why is an additional rcd needed in all cases

Well believe it or not its not there are people who out there who dont adhere to such high standards, theyll whack a 3/8 twig in on top of a mountain and leave with there big cheque, have seen in numerous occasions out in rural areas!

 

[Simonslimline]

Here is one of mine i did a while back if you want me to go first, And yes i am aware of the crud by the main switch etc. It's not there now.

 

[Loki]

Nice very neat Simonslimline

loki