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

 

[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.

 

[pc1966]

This makes sense now as in TT system we have higher earth resistance so we cannot rely on ADS to occur hence we need time delay 100mA RCD for fault protection and then for higher risk areas 30mA RCD for additional protection.

Strictly speaking it is always ADS as the supply is automatically disconnected on a fault.

The difference is on a TN system then usually you can achieve it on the over-current protection device (fuse, MCB, etc) due to low Zs if a reasonable design permits it, but on TT it almost always has to be an RCD due to the relatively high Ra (and so high Zs and low PFC).

But you can get situations on TN with high current final circuits or sub-mains where it is not feasible to use OCPD to meet the disconnections times and so then you are back to using an RCD in combination with OCPD. For many final circuits that can just be an RCBO, but for sub-mains you might be looking at fuse-switch or MCB + delay RCD combinations, or fancy (and expensive) MCCB that feature adjustable earth leak trip settings, etc.

 

[pc1966]

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.

My concern is the RCBO tripping on any surge and nobody notices the SPD has not been isolated, then next surge and some hardware gets broken.

An up-front delay RCD should ignore a short spike anyway, and if it did trip on a failed-to-short SPD then at least you find out. I suspect most SPD would fail open, as they usually have solder joints that are designed as ultimate thermal disconnection, but I would never rule out a short-fault as impossible.

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.

The use of a proper tail gland and the 19-strand flexible tails should avoid any risk of a short before the RCD/RCBO but I can see the attraction of an up-front RCD being in its own insulated enclosure.

Many of those rules are to try and reduce the inevitable impact of rubbish workmanship:

• A well-installed metal CU should be perfectly safe on TT
• A a well-installed plastic enclosure for RCD the same against fire

But given the possibility of SPD getting very hot under major faults (lightning hit to pylon/building, or worst-case open-PEN allowing ~400V L-N) I personally would only ever fit SPD in non-combustible locations.

 

[Aaron b]

Would it be?

Double pole isolation is required under 462.2, and as you mentioned earlier disruption should be avoided by separating circuits. Since double pole breakers are practically attainable what would be the reason not to install them?

It might be worth noting it was an earlier version of the regulations that said that an upfront RCD was required with a class 1 CU (before the change to metal cu's in dwellings). Now it mentions double insulation of the conductors supplying the RCCB. That might make split load boards harder to use on TT systems and a cheep board upgrade might not comply.