Stroma Certification Scheme What RCD for 18th Edition

[Michaelwgroves]

I'm a bit late taking my 18th edition update, I've booked for end of November and have my assessment early December.
I'm gonna do a Consumer unit upgrade for assessment. I've not done one since 18th edition, been a bit quiet.
So I'm looking at RCD's, I typically fit Hager, and their pre-built Consumer units come with two Type 'A'. This covers most domestic scenarios, but when you have a fixed load like ovens with resistive heating element, this should be type 'AC' surly. Assuming most houses will have at least an electric oven, maybe immersion heater and electric shower. I should have thought split boards would have 'A' and 'AC' so you can choose appropriate protection.
Also 17th edition was type 'B', why have we moved away from this?
I know it'll all get covered on the course, but want to get ahead. I would be grateful for any input.

 

[Julie.]

In practical terms, the only advantage a type AC rcd has over type A is that it may cost a little less. Although that price difference is almost eliminated now.

You can use type B but they are much more expensive generally, although there's advantages if you have pure dc, but unless you need to support specialist applications, type A would cover most things.

Why would you want to use AC in preference to type A?

 

[kropaske]

Type A rcd do same as AC and additionally trip on pulsating DC up to 6mA.
Look into SPD requirements when changing the board as this may be mention on the assessment

 

[buzzlightyear]

any body started putting in A rated yet
Considering the IET did not put that for ward and even 18 th been out
and not at the my wholesalers Yet.
the reason I'm asking a chap at the wholesalers screaming I need them now.

 

[Michaelwgroves]

In practical terms, the only advantage a type AC rcd has over type A is that it may cost a little less. Although that price difference is almost eliminated now.

You can use type B but they are much more expensive generally, although there's advantages if you have pure dc, but unless you need to support specialist applications, type A would cover most things.

Why would you want to use AC in preference to type A?

Actually, having read 531.3.3 (i) & (ii) again properly, I can see 'A' includes everything in 'AC' so in fact, I would not choose 'AC' in preference to 'A'

Thanks for that, I've leant something already.

 

[Michaelwgroves]

Type A rcd do same as AC and additionally trip on pulsating DC up to 6mA.
Look into SPD requirements when changing the board as this may be mention on the assessment

I'm going to fit SPD, from what I can see its less work to fit, rather than not. I also want the experience.

 

[Michaelwgroves]

any body started putting in A rated yet
Considering the IET did not put that for ward and even 18 th been out
and not at the my wholesalers Yet.
the reason I'm asking a chap at the wholesalers screaming I need them now.

A rated, what's that about?

 

[buzzlightyear]

Switching on specific appliances or a combination of electrical loads such as LED lighting can produce electrical transients in the order of several 1000 amps for a very short duration. These transients can cause nuisance tripping of standard non-delay Type AC and A RCDs. To overcome this problem, RCDs including 30mA devices, can be manufactured with transient resistant features- see right hand column in the table. Type AKV, F, B and EV RCDs can withstand a fast transient <3kA/20µs. e.g. the Regulations recommend the use transient resistant RCDs, if the circuit contains surge protection devices (SPDs) on the load side of the RCD.

 

[Michaelwgroves]

the Regulations recommend the use transient resistant RCDs, if the circuit contains surge protection devices (SPDs) on the load side of the RCD.

Would the SPD not be on the supply side in most scenarios?

 

[pc1966]

Yes, the SPD is normally on the main bus-bar, though some SPD require a MCB as well.

Generally go with SPD that are OK for 125A or more fuses, as they don't use additional CU slots for any MCB and usually work better (as the MCB has impedance that degrades the SPD effectiveness on short pulses).

 

[Julie.]

Would the SPD not be on the supply side in most scenarios?

Yes, as said above, you don't really want a rcd/rcbo upstream of a spd; the spd works by shunting current neutral-earth/cpc and line-earth/cpc so the rcd could operate.

The rcd wouldn't operate quick enough to inhibit the operation of the spd, but would remain tripped after it, so a subsequent surge wouldn't be protected against by the spd as it would be out of circuit.

Most spd are direct on the busbar being protected by the main incoming fuse, or have a dedicated mcb or fuse (unusual these days) within the cu.

Fuses are more common on large industrial sites where the incoming fuses are several hundred amps and three phase.