RCD protection for TT system

[Big_Spark]

Adam..that WAS NOT BAS's point, and even if it was it is wrong..single module, often referred to as single pole, RCBO's ARE double pole devices...

 

[PompeySparks]

Yes but when a single module RCBO is used, the circuits neutral is still connected straight to the neutral bar so any neutral to earth fault on that circuit it will still affect other circuits connected to that neutral bar as nothing physically removes the offending circuit's neutral bar/earth bar connections in the event of a fault! This is why a N-E fault would still cause a problem! Strictly speaking a single module RCBO is NOT double pole as it doesn't break the circuits live and neutral between the source of supply and final load in the event of a fault.

 

[ban-all-sheds]

Adam..that WAS NOT BAS's point, and even if it was it is wrong..single module, often referred to as single pole, RCBO's ARE double pole devices...

That was my point, and no I am not wrong.

If I am wrong, why do MK describe their single-module RCBOs as single-pole with solid neutral?

If I am wrong, why do Crabtree describe their single-module RCBOs as single-pole with solid neutral?

If I am wrong, why do Siemens describe their single-module RCBOs as 1 pole plus a solid neutral?

(Don't have any more technical manuals to hand)

BAS..you dont not know how the kit works, thast is obvious, single Pole RCBO's are IDENTICAL in their operation to double pole RCBO's..and please don't try to argue with me as on this point I will win the arguement...

FFS, here we go again.

I really don't have the stomach for this today - could somebody else please carry on the argument until he either claims that we all misunderstood what he said, or that he was simply relaying what the manufacturers had told him without ever actually thinking about it, or he just goes all quiet?

 

[Big_Spark]

Pompey..I don't know whose RCBO's you've been using but the circuit Neutral goes to the RCBO, the RCBO has a coiled fly lead that then connects to the Neutral Busbar..When the RCBO trips it disconnects both the phase and Neutral connections...

 

[PompeySparks]

Apologies Big_Sparks I misread your earlier comments and thought you had found an RCBO that only connects to phase (and somehow monitors Neutral via flylead!) - it doesn't make much sense but I know but I thought that's what you meant - my mistake! My main point still remains however - If the connection inside the RCBO is solid neutral then

1.) the RCBO would not break the neutral connection from the offending circuit to the neutral bar - therefore current would still be lost to earth in the event of a N-E fault even after the RCBO had triggered, thus tripping the main RCD incomer (TT system) as well

2.) The device would not be classified as a double pole device as the neutral is never broken in the RCBO

But sorry for misreading the other part!

It'll all come down to the 'solid neutral' debate - if the RCBO DOES disconnect L & N as you state then you would be totally right but I don't think it does in the 'single pole/module' case

 

[Big_Spark]

BAS...do you remember the technical gear on my website..If you recall I took single phase and three phase MCCB's, MCB's, RCD's and RCBO's apart and photographed them.

The single phase units were both single module and twin module..both, if you recall, had double pole contacts in them...What would be the point if they were single pole devices??

However I have not installed any Crabtree kit in a long long time so would not comment on that kit.

I have also looked at the MK site and agree that they do say their kit is single pole..so it must be..

Perhaps this difference is because I stick to MEM and Merlin-Gerin, and they are the kit I took apart for the pictures...

I have checked the Eaton MEM website and they make them as SPSN (Single Pole Solid Neutral), SP (Single Pole), and SMDP (Single Module Double Pole), the difference being in the part number..Obviously..

According to the Merlin-Gerin Catologue I have here, all their single module RCBO's are Double Pole, I presume this is due to being focused on heavy Commercial and Industrial applications.

So we are both correct on this.....I have never bought single pole devices as I have always quoted the same part numbers.

Apologies to all if my earlier comments were misleading, it was not intentional.

 

[BSBS]

Again, if I understood BSBS, he was querying the use of a 300 instead of the more normal 100, not a 30.

Indeed i am since every circuit in a TT installation must be protected by an RCD and must comply with regulation 413-02-20

the condition shall be fullfilled for each circuit:

Ra * I delta n <50v

so although in this instance having an RA of 140ohms and a 300ma RCd as you suggested does come out at 42V is below the 50V maximum allowed it is very close and could easily go above 140ohms come the summer.
I am not saying its wrong i am merley pointing out that it is perhaps a little to close to the maximum Value and a smaller RCD would be a better option.

A 300ma will only offer the correct protection upto 166ohms anything above this will require a lowr rated RCD


It may also be worth noting that for an agricutal and building sites that the formula changes to Ra * I delta n < 25V

 

[Big_Spark]

BSBS..I understand thepoint your making, but to be honest I have never settled for an installation has a reading above 100 Ohms..in fact in the not too distant past I drove 8 Earth rods into the ground..obviously joined together..until the probe resistance was down to a mere 12 Ohms...

I know the Regs allow us to accept a reading up to 100 Ohms (NICEIC) or 200 Ohms for BS7671 (If I recall correctly), but I am never happy with this as, as you rightly point out, conditions change, so the lower it is to start with, the lower it should be when it changes...

 

[fastJedi]

I managed to find some Volex product information on what I assume is the UK Distributors web site (Electrium Sales Ltd) They don't appear to supply an RCD that will allow any kind of discrimination (lesson learnt ). Assuming I am unsuccessful in my PME request ... and am determined to uphold my cheap skate reputation .... Is it O.K to put an RCD in a separate enclosure infront of my consumer unit?

PS: I think I'll also add another earth spike in view of this discussion.

 

[Big_Spark]

Fastjedi..Yes you can add a seperate RCD in a seperate enclosure, this would be connected between your electricity meter and your consumer unit..

Regarding the installation of a new Earth spike, ensure you know where your drains and any underground pipes, cables are..you don't want to go creating a bigger problem...

Regarding the Earth spike, I would suggest that first you check the one you have, it may be adequate for your purposes.

Failing that, employ a local electrician to do this for you..or doa lot of reading on the subject first..this is something I am sure you can do, but it must be done right first time.

 

[ban-all-sheds]

So we are both correct on this.....I have never bought single pole devices as I have always quoted the same part numbers.

Apologies to all if my earlier comments were misleading, it was not intentional.

So are we all agreed then that:

1) Whether it occupies 1 or 2 modules, you must use a Double Pole RCBO or you will not get discrimination on N-E faults.

2)This:

BAS..you dont not know how the kit works, thast is obvious, single Pole RCBO's are IDENTICAL in their operation to double pole RCBO's..

is wrong.

3)This:

and please don't try to argue with me as on this point I will win the arguement..

was yet another example of your aggressive pig-headedness, so often displayed when people disagree with you?

 

[Big_Spark]

Yawn....

 

[andy]

have to admit i havent came accross double pole RCBO's (but then i dont work with them often). would be a bit tight for space tho having to break 2 contacts

 

[ebee]

Far be it form me to say it but.........................................................................
Ban wins again

 

[Big_Spark]

Far be it form me to say it but.........................................................................
Ban wins again

It's not a competition and no BAS did not win, niether did I.