How does Touch voltage work without RCD protection

[King84]

Hi everyone
I asked my assessor about touch voltage and he was not keen on explaining properly and gave me a rough answer which made me think probably he was not sure himself or was hiding it.

basically my understanding is afcourse that how 30mA RCD works to limit touch voltage to 50V is by 50/0.03 as this gives enough 1667ohms resistance margin so as long as resistance is within this limit, the voltage will remain under 50V.

My question was with assessor as in TT system as we do need to have 100mA RCD afcourse for fault protection as due to low fault current and higher Ze but what if we dont have 30mA RCD for additional protection then what gives additional protection in that situation and upon hearing this, the assessor just gave me Touch volatage will be limited to 50V as use the formula so that got me confused how Ra=50/Ia so lets say we have higher resistance due to which we will be having lower fault current then how additonal protection is acheived in that scenario ? am I missing out something

please if anyone could briefly explain how touch voltage occurs with RCD and without RCD on normal MCBs how touch voltage is acheived? thanks tons for your great help

 

[King84]

It's good practice to install a 100mA type S upfront but not a requirement of bs7671. A single 30mA can serve both fault and additional protection.

Thanks Aaron but if 30mA can provide Fault and additional protection then why 100mA RCD is required upstream?

 

[Aaron b]

Thanks Aaron but if 30mA can provide Fault and additional protection then why 100mA RCD is required upstream?

It's not but most would feel safer having two devices.

 

[davesparks]

It's good practice to install a 100mA type S upfront but not a requirement of bs7671. A single 30mA can serve both fault and additional protection.

Since when has that been good practice?

I can't see any logic to it other than inviting unnecessary nuisance tripping.

 

[timhoward]

It's good practice to install a 100mA type S upfront but not a requirement of bs7671. A single 30mA can serve both fault and additional protection.

You often find older 5 way consumer units with an 30ma RCD main switch providing fault protection and additional protection. (These are the least likely RCD's to ever be tested and I've known quite a few not work)
It's worth adding that for a few reasons it's good if each circuit (or at least group of circuits) have it's own RCD.
a) with the general increase of leaky equipment, if you gather all the leakage of every circuit together this can increase nuisance tripping.
b) with the increase in the amount of DC leakage it's good not to gather it all together in case it blinds the RCD and it fails to operate.
c) It can help if there's a fault to know which instantly know which circuit or group of circuits is the likely culprit. It isn't always this simple but it's a good start.

Most of the time these days every circuit has it's own combined MCB and RCD (RCBO) 30ma protection and this is acceptable for TT installations.
If the consumer unit is a older split load board, with some circuits NOT RCD protected, then the main switch would be an S-Type 100ma RCD on a TT system, or there would be a similar RCD in it's own enclosure before the board.

 

[Aaron b]

Since when has that been good practice?

I can't see any logic to it other than inviting unnecessary nuisance tripping.

I suppose it each to there own when going over and above spec. Your right with considering nuisance tripping with the increased earth leakage from electronics.

 

[davesparks]

I suppose it each to there own when going over and above spec. Your right with considering nuisance tripping with the increased earth leakage from electronics.

I have no issue with going over and above to improve a situation if it is genuinely well thought out and actually improves a situation. However you have not presented this as a way of going over and above, you presented it as good practice, this implies that the industry as a whole approves of it rather than being your personal opinion.

I still fail to see the point of installing an extra 100mA RCD on a normal TN installation.

A 100mA RCD does not provide additional protection, it will allow a fatal electric shock before it operates so does not provide any back-up to a 30mA RCD. It may however provide a false sense of security for someone who doesn't understand this.

For earth fault protection you already have multiple means of detecting those faults either by MCB and 30mA RCD or 30mA RCBO.

Nuisance tripping will occur with neutral to earth faults because RCBO's (assuming they are used) generally do not disconnect the neutral, so the 100mA RCD will trip as well as the 30mA RCBO. This will defeat the whole object of dividing the installation up across multiple RCDs. Again, in my opinion, it gives a false sense of security whilst having the potential to create a larger problem.

 

[nicebutdim]

I still fail to see the point of installing an extra 100mA RCD on a normal TN installation.

I may have mis-read the comment, but my take was that the suggestion was in relation TT installations.

While not required, it does add an additional layer of protection where ADS may rely entirely upon on electronic components.

 

[Aaron b]

I still fail to see the point of installing an extra 100mA RCD on a normal TN installation.

The OP was referring to a TT installation. I'm assuming he has come across the 100mA type S and was just trying to provide some insight to why it might have been done...

 

[King84]

Dave so if on TT assuming we are not using RCBOS but splitboard RCD 30mA and for mainswitch 100mA then neutral is no longer an issue so for selectivity time delayed RCD 100mA is a good choice alongside with 30mA RCD ?
Sorry I am just trying to understand why on existing so many TT people are using 100mA if it will cause nuisance tripping and as you mentioned RCBO scenario so I am assuming this is the main reason ?

 

[davesparks]

Dave so if on TT assuming we are not using RCBOS but splitboard RCD 30mA and for mainswitch 100mA then neutral is no longer an issue so for selectivity time delayed RCD 100mA is a good choice alongside with 30mA RCD ?
Sorry I am just trying to understand why on existing so many TT people are using 100mA if it will cause nuisance tripping and as you mentioned RCBO scenario so I am assuming this is the main reason ?

Yes if double pole 30mA RCDs are installed, or DP/SPSN RCBO's, then selectivity will occur on either a L-E or N-E fault with an upstream 100mA S type RCD.

If on a TT system you have circuits which do not require additional protection at 30mA then a 100mA time delayed RCD would be used to provide fault protection.
A 100mA time delayed RCD may also be installed to provide protection for the wiring within the DB which is not protected by the RCBO's etc.


With a TT system in some respects you have to choose the lesser of 2 evils as far as the neutral to earth fault problem goes, the tripping problem still exists but the danger of not having a 100mA RCD can outweigh the danger of tripping causing an entire installation to go off.

Also consider how much diverted neutral current there will actually be in a N-E fault on a TT system.

Why you will find a lot of TT systems with a 100mA RCD mainswitch and all RCBO's after it may well be due to them being installed by people who blindly follow a book rather than thinking, or people who just don't bother to keep up to date with their knowledge, or even people who woukd rather listen to gossip and rumours than actually read and understand the rules.

 

[nicebutdim]

Why you will find a lot of TT systems with a 100mA RCD mainswitch and all RCBO's after it may well be due to them being installed by people who blindly follow a book rather than thinking, or people who just don't bother to keep up to date with their knowledge, or even people who woukd rather listen to gossip and rumours than actually read and understand the rules.

Given the fact that RCD failure isn't particularly uncommon, would you consider additional 100mA Type S protection to be unwarranted for TT systems?


Edit: This assumes the installation of DP RCBOs

 

[Aaron b]

I think it's fair to assume double pole breakers would be used on a new TT installation as it would be needed for isolation, right?

 

[davesparks]

I think it's fair to assume double pole breakers would be used on a new TT installation as it would be needed for isolation, right?

Would it be?

 

[davesparks]

Given the fact that RCD failure isn't particularly uncommon, would you consider additional 100mA Type S protection to be unwarranted for TT systems?


Edit: This assumes the installation of DP RCBOs

I don't consider it to be unwarranted on a TT installation.

Personally if I was installing a domestic TT installation I would prefer to install a 100mA S type main switch and SPSN RCBO's for all circuits.
I'd also install the 100mA RCD in a separate insulated enclosure as I feel it is a justifiable departure to do so. I'd probably use the enclosure from a REC2 or similar as they clamp the tails quite nicely and are pretty sturdy.

This is of course personal opinion

 

[timhoward]

Why you will find a lot of TT systems with a 100mA RCD mainswitch and all RCBO's after it may well be due to them being installed by people who blindly follow a book rather than thinking, or people who just don't bother to keep up to date with their knowledge, or even people who woukd rather listen to gossip and rumours than actually read and understand the rules.

All agreed. I'm not quite ready to say that are no circumstances with an RCBO board where it makes sense though.
I know you've mentioned single vs double pole RCBO's, but if double pole are used it's a legitimate 2nd chance at fault protection should for whatever reason the downstream device not operate.

It covers some rather extreme scenario's that I accept should not happen in the first place such as a bus bar to case fault or someone slipping with a screwdriver.

Finally several brands of CU have an SPD kit that is fed from an MCB. It's highly unlikely, but a faulty SPD would otherwise have no fault protection. I've not had to consider this one yet, and whether I'd change the MCB for an RCBO etc.

I also prefer an up-front enclosure as whatever the regs say I've never liked metal board + TT.
It used to be such an absolute no-no and very quickly became fine as long as a tails gland and clamp is used.