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

 

[timhoward]

I think you might be chucking too many different things into the same blender.
You need fault protection. That is achieved either through low impedance to earth so there is sufficient fault current to cause ADS or detecting leakage to earth using an RCD.
On TT systems the touch voltage shall be limited exactly as you said.
On TN systems ADS should occur instantly when a fault occurs so the touch voltage concept isn’t particularly relevant in most cases.

The choice of RCD technology is a separate matter again, and the general rule is that for discrimination you need X3 upstream. So if individual circuits are protected by 30ma RCDs which the regs require for many things then you need >90ma so in reality 100ma RCD to cover the entire installation.
If you only have the 100ma RCD for fault protection then that isn’t meeting the requirements of the regs for additional protection of E.g. socket circuits or domestic lighting circuits.
(Some older TT installs have a main switch RCD rated at 30ma.)
Does that help at all?

 

[davesparks]

The choice of RCD technology is a separate matter again, and the general rule is that for discrimination you need X3 upstream. So if individual circuits are protected by 30ma RCDs which the regs require for many things then you need >90ma so in reality 100ma RCD to cover the entire installation.

Just having a tripping current of x3 doesn't give you discrimination with RCDs, you need a time delay long enough to allow the downstream RCD to operate before the upstream one does.

 

[timhoward]

Just having a tripping current of x3 doesn't give you discrimination with RCDs, you need a time delay long enough to allow the downstream RCD to operate before the upstream one does.

Sure, I’m on holiday typing on my phone….
I should indeed have said S type or R type
or manually configurable for the upstream device. ( X3 I delta N or above also needed)

 

[pc1966]

You also have to remember that RCD do not limit the current, or by implication limit the voltage. What they limit is the exposure time at currents above the trip threshold.

The maximum values of Ra typically given from 50V/In for the RCD are the upper limit of resistance on a running system with a controlled leakage of current that in not guaranteed to trip the RCD and which will keep the continious 'touch voltage' below 50V. In earlier versions of the regs I think it was reduced 25V for agricultural areas, etc (i.e. half the resistance).

That is why you can't simply use RCD in very hazardous areas, such as in or very close/in a bath or swimming pool, since your RCD disconnection time might still allow a seriously high shock current through a wet conductive body that can kill or cause injury even if it only lasts for tens of milliseconds. Hence you use ELV there.

 

[King84]

I think you might be chucking too many different things into the same blender.
You need fault protection. That is achieved either through low impedance to earth so there is sufficient fault current to cause ADS or detecting leakage to earth using an RCD.
On TT systems the touch voltage shall be limited exactly as you said.
On TN systems ADS should occur instantly when a fault occurs so the touch voltage concept isn’t particularly relevant in most cases.

The choice of RCD technology is a separate matter again, and the general rule is that for discrimination you need X3 upstream. So if individual circuits are protected by 30ma RCDs which the regs require for many things then you need >90ma so in reality 100ma RCD to cover the entire installation.
If you only have the 100ma RCD for fault protection then that isn’t meeting the requirements of the regs for additional protection of E.g. socket circuits or domestic lighting circuits.
(Some older TT installs have a main switch RCD rated at 30ma.)
Does that help at all?

Thankyou very much for detailed reply and it all makes sense but I have one more question thats bothering me and couldnt find the answer.

Lets say RCD operates in 40ms at 5 times to give aditional protection which means accidently someone touches live cable , RCD operates in 40 ms at leakage of 30mA current which will obviously protect from electric shock.

My question is there are many old installations which obviously are still installed and it is said because they are old doesnt mean they are unsafe.
If anyone accidently touches live cable and considering Zs are all below 80% then I do understand MCB would operate within 0.4s but would that prevent electric shock?
I thought maybe touch voltage play any part here .sorry but I am still trying to grasp this part and i appreciate your help

 

[pc1966]

My question is there are many old installations which obviously are still installed and it is said because they are old doesnt mean they are unsafe.
If anyone accidently touches live cable and considering Zs are all below 80% then I do understand MCB would operate within 0.4s but would that prevent electric shock?

Without an RCD if someone touches the cable it will not disconnect.

The Automatic Disconnection of Supply (ADS) that has been in the regulations for decades is about a fault live to earthed metalwork (i.e. to the CPC) with someone in contact with the CPC (i.e. that metal object) and also the true Earth (i.e. defined zero voltage).

That is what the Zs "fault impedance" is all about, that if you get a fault L-E then so much current flows that the MCB or fuse disconnects in under 0.4s (for a TN supply, nominally 230V) or 5s for sub-main.

If you have a TT system, so the CPC goes to an earth rod, then the disconnection times drop to 0.2s and 1s respectively.

Why? Well in the TN case your R1 and R2 are usually of the same order, so a L-E fault causes roughly half the supply voltage, say 115V to appear during a fault as the voltage drop on R1 and the voltage rise on R2 are about the same. Whereas for a TT system R2 is usually very much bigger than R1 so under fault conditions almost all of the 230V appears on your metalwork, and so the disconnection times have to be less so the shock risk is less since the unfortunate handler of the metalwork gets about* twice the current for a given sweaty palm's resistance, etc.

Also the TT level of 'R2' is usually so high that even a 6A MCB won't trip. Ever. You are usually reliant on a RCD for fault protection, and that traditionally for domestic supplies was a 100mA delay RCD as an incomer along with possibly a 30mA 'instant' RCD for high risk circuits like socket outlets. High risk in that prior to RCDs being the norm and cheap-ish, every summer a dozen or so folks in the UK would die from mowing an extension lead, etc, in the garden.

As already mentioned above when you have RCD in series and want selectivity you need each level up-stream to be around x3 higher threshold current, as well as about an extra 0.2s delay so a big fault can be isolated downstream if possible, and if not then shortly after the next level up goes. Few systems have more than a 300mA/100mA S-type then 30mA instant, but if you had a larger system then the next level up would typically be an adjustable RCD set to 1A/300mA and about 0.4s delay.

[*] Human body resistance is not linear, above a certain voltage it drops making life harder and potentially very much shorter.

Electrical injury - Wikipedia

en.wikipedia.org

 

[Aaron b]

Thankyou very much for detailed reply and it all makes sense but I have one more question thats bothering me and couldnt find the answer.

Lets say RCD operates in 40ms at 5 times to give aditional protection which means accidently someone touches live cable , RCD operates in 40 ms at leakage of 30mA current which will obviously protect from electric shock.

My question is there are many old installations which obviously are still installed and it is said because they are old doesnt mean they are unsafe.
If anyone accidently touches live cable and considering Zs are all below 80% then I do understand MCB would operate within 0.4s but would that prevent electric shock?
I thought maybe touch voltage play any part here .sorry but I am still trying to grasp this part and i appreciate your help

That's not the point of ads. Basic protection is to stop contact with conductors which used to be referred to as direct contact. Ads for fault protection or indirect contact, it's also all based on a single fault. Having all exposed-conductive-parts all connected to the MET should help with the temporary rise in potential.

 

[davesparks]

My question is there are many old installations which obviously are still installed and it is said because they are old doesnt mean they are unsafe.

Not quite, there is a line in the regulations that says installations carried out to previous versions of the regulations may not necessarily be unsafe.
This does not mean that all old installations are automatically safe, it means that they may or may not be.

If anyone accidently touches live cable and considering Zs are all below 80% then I do understand MCB would operate within 0.4s but would that prevent electric shock?

If someone touches a live cable the MCB won't operate because of that alone, or if it does they will be very much dead.

Also, RCDs don't prevent an electric shock, they reduce the severity of the shock.

 

[LastManOnline]

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.

Touch voltage is a very simple concept when you look at it from the perspective of ohms law.. current flowing in the circuit multiplied by the resistance (earth rod in TT system). This voltage will be present on all earthed /bonded metalwork connected to the rod.

basically my understanding is afcourse that how 30mA RCD works to limit touch voltage to 50V

The rcd has no affect on the "touch voltage" whatsoever. It simply limits the time that voltage is present on the affected metalwork.

please if anyone could briefly explain how touch voltage occurs with RCD and without RCD on normal MCBs how touch voltage is acheived?

Again, the safety devices have no ability to limit the touch voltage. Their purpose is to limit the time factor that said voltage will be present on affected metalwork or persons.

thanks tons for your great help

You are welcome