RCD curve type

[RickH]

Scenario … a pair of wooden outbuildings, are fed from an 30mA RCD in the house, from the outbuildings a small consumer unit divides it up … couple of AC sockets and lights in out buildings. Then an armoured cable out to a decking area for a 32A hot tub.
The HT has local earth rod.
Previously outbuildings were a TT circuit to local earth rod, house has now been converted to PME and now earth is exported to outbuildings, but not to Hot Tub which remains on its own local earth rod, and protects armours as far as outbuilding.
Sketch attached may help - MCB interlinks etc. left out for simplicitiy
All has been tested and Part P certified by Electrician.

Today he added a comment that the RCD is curve AC, and I should consider changing it to curve A, for DC protection.
Would like to understand why, he mentioned DC protection, but there are no DC circuits - unless he meant leakage currents.




So for this situation I should change to Type A

 

[Deleted member 142255]

I think you are confusing MCB's and RCD's.

 

[RandomGrinch]

Might be have said your RCD is type AC, and it should be type A?

 

[RickH]

I just went and looked my RCD is definitely Type AC
The symbol on it is as per:

The note he left shows to change to type A.

So I incorrectly compared A and C … Should have compared A and AC (have updated)

 

[RickH]

Might be have said your RCD is type AC, and it should be type A?

I just looked at note I have- that is correct.
So swapping to type A is a better option ?

 

[flameport]

Type AC for resistive loads only, and with modern equipment that means pretty much nothing.

Type A is the minimum.
Some devices may require other types such as F or B.

 

[RickH]

I think you are confusing MCB's and RCD's.

No it was relating to RCD

 

[Deleted member 142255]

No it was relating to RCD

Your terminology was confusingly incorrect, that was what I was querying.

 

[pete01]

Rcds don't have curves...mcbs have curves...rcds have types.

 

[RickH]

So swapping out to a type A is a good idea.

 

[RickH]

Rcds don't have curves...mcbs have curves...rcds have types.

OK electrician here today referred to them as curves …. I just followed that.
However the Q is Type A or Type AC. …. Understand now that is the correct terminology … Thnx

 

[pete01]

OK electric Ian here today referred to them as curves …. I just followed that.
However the Q is Type A or Type AC

Well as far is I know, type AC can no longer fitted but you've already got them so cant be forced to change them but he is advising you which is good advice (you don't really want something that isnt allowed to be fitted these days)

Type AC have been banned in other countries including Germany...I think.. for many years.

 

[RickH]

Might be have said your RCD is type AC, and it should be type A?

Agree … my error

 

[JohnW2]

Type AC for resistive loads only, and with modern equipment that means pretty much nothing. .... Type A is the minimum. .... Some devices may require other types such as F or B.

All true. However, to put this issue into perspective for the OP (and others), his Type AC RCD is in the very good company of countless millions of Type AC RCDs currently in-service in the UK.

After all, it is only a couple of years since the UK Wiring Regulations (BS7671) stopped saying ...

For general purposes, Type AC RCDs may be used.

... and the great majority of RCDs currently in service in the UK were probably installed prior to that statement becoming 'obsolete'.

 

[JohnW2]

OK electrician here today referred to them as curves …. I just followed that. However the Q is Type A or Type AC. …. Understand now that is the correct terminology

It's primarily with RCBOs that there is potential serious (and, I would say, ridiculous and unnecessary) confusions.

RCBOs, being the combination of an MCB and an RCD, cave what we now call both a 'Curve' (B, C or D) and a 'Type' (A, AC, B or F).

In relation to MCBs, and probably also RCBOs (which were, in the UK, then probably nearly all what we would now call "Type AC") , until pretty recently everyone used "Type" to refer to what we now call "Curves" - hence what was then called a "Type B" device was one which had a "B curve" [Even today, I think that there are places in the Wiring Regulations (BS7671) which still use such terminology] but then, seeming all of a sudden, "Type" was being used to relate to a totally different characteristic of RCDs and RCBOs.

I seem to recall that at least one of the regular participants in this forum was caught out by that, buying what he believed to be a "Type B" RCBO, only to find that it was actually a B-Curve Type AC one!

Since there are no 'A', 'AC' or 'F' "curves", this potentially serious confusion only exists in the case of Type B vs B-Curve RCBOs.