TT ,17th and TYpe S RCD

[binky]

Given that new split boards have twin 30mA RCD covering all circuits, do I still need 100mA type S as incoming main switch?? Have dug deep into regs and guide book and issue is not clear, except the one diagran in onsite guide does show standard TT setup as per 16th regs with no additional 30ma RCD for lighting???

 

[kme]

No; you don`t. In fact, technically, you never did. you could have used two rcd`s in the board before; as long as BOTH were fed from the main switch; rather than chained.

The new boards have a main switch; and two RCDs. If you`re on a TT; only populate the RCD sections; unless you`re going for RBCO`s (my personal preferred option).

 

[binky]

That's indd of what I thought KME, but a phone call to NICEY bloke said otherwise, hence I posted question because I can't see the point in protecting RCDs with a higher rated RCD. It just doesn't make sense - so what's new there then

 

[kme]

Why? What did they say? Did they give any reg. Nos to back it up??

 

[steptoe]

NOT required,

unless as a personal preference you want to add some protection to your circuits that dont require 30mA protection,

ie, FCU etc, washing machine/fridge.

 

[binky]

If you look at table 41.5 in regs it's interesting that you could have a a Zs of upto 1667 ohms with a 30mA RCD albeit with the proviso that values exceeding 200 ohms may not be stable.

 

[steptoe]

i dont even think u need a 30mA with them readings if you calc it,

its usually the DNO that limits what they will accept.

normally <180 I think

 

[binky]

You need an RCD because you can't meet Zs values as you would for TN-S or PME, its under section 411.5 that RCD gives earth fault protection. What I've always wondered is if the rod was 50 ohms, does this mean you could have a resistance (R1+R2) of 150 ohms on the circuit (I believe the answer is no but its not very clear from reading regs and guide books)

 

[batty]

Hello Binky,

There is no need for td rcd as main switch as all circuits have are rcd protected on the 17th edition boards. Although some of the boards like mk's 15 way 17th edition board has provision for 4 circuits before rcd that could either be used for mcb's or rcbo's. I used one recently on a tt installation. I put oven on rcbo, hob on rcbo, kitchen ring on rcbo and utility ring on another rcbo. The remaining circuits split between other rcd's. The only problem I could see using one of the boards on a tt system is if you had not used all four unprotected circuits and then an Unqualified person where to put in an mcb rather than an rcbo for a new circuit, but with part p this should not happen aha.

batty

 

[kme]

Hello Binky,There is no need for td rcd as main switch as all circuits have are rcd protected on the 17th edition boards. Although some of the boards like mk's 15 way 17th edition board has provision for 4 circuits before rcd that could either be used for mcb's or rcbo's. I used one recently on a tt installation. I put oven on rcbo, hob on rcbo, kitchen ring on rcbo and utility ring on another rcbo. The remaining circuits split between other rcd's. The only problem I could see using one of the boards on a tt system is if you had not used all four unprotected circuits and then an Unqualified person where to put in an mcb rather than an rcbo for a new circuit, but with part p this should not happen aha.

batty

But would that actually be YOUR concern? ANY diy joe can go and do something stupid after you. I`ve had someone go into one of my jobs; and swap an rcbo for an mcb, `cos he "needed" an rcbo on another job!!!!!

Nothing I can do, other than move his chewing equipment nearer his rectum, but that doesn`t help the customer. I know it sounds hard, but coertify your work properly, and walk away.

 

[steptoe]

But would that actually be YOUR concern? ANY diy joe can go and do something stupid after you. I`ve had someone go into one of my jobs; and swap an rcbo for an mcb, `cos he "needed" an rcbo on another job!!!!!Nothing I can do, other than move his chewing equipment nearer his rectum, but that doesn`t help the customer. I know it sounds hard, but coertify your work properly, and walk away.

TOTALLY AGREE,

as for 17th, its still not a requirement for 30mA protection, if you provide other protection,

way back when, and still today believe it or not,

500mA is adequate protection for a TT system,

tho, TBH I always use a 100mA as a main switch on TT.

too many people are reading the regs as law,

they are only a guide, (jez Im fed up saying that(watch my signature))

i personally have had lots of digs from a 30mA without it tripping, P-N, treats u as a kettle element, only detects leakage to earth so on a 1667 resistance would be negilable, but the overcurrent wopuld be instantly cut off,

choose your MCB/RCBO wisely.

 

[batty]

I agree as long as you have filled in your certs and where the spare ways you have marked them as spares you should have no probs.

batty