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

 

[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.