How does Touch voltage work without RCD protection

[King84]

Thanks alot everyone, you guys have been very helpful I really appreciate. I wish my books could explain the way you guys explained.

This makes all sense now RCD does not have impact on the current but effects time exposure and ADS is caused by fault current so it has nothing to do with touch voltage.

I am only wondering like you guys mentioned just because regs says as older installations have no RCD does not make them necessarily unsafe so I understand as ADS is there which is caused by L-E . afcourse along side with main protective bonding etc.

If there is a metal frame light switch and property is old with no RCD and there is some contact between Line conductor and earth or lets say metal work, in the situation how would the disconnection is acheived to prevent electric shock?

MCBS only provide overloading and short circuit protection but would ADS provide protection in the situation mentioned ?

sorry guys I am still in learning stage , maybe I am overthinking about it

 

[Aaron b]

I am only wondering like you guys mentioned just because regs says as older installations have no RCD does not make them necessarily unsafe so I understand as ADS is there which is caused by L-E . afcourse along side with main protective bonding etc.

If there is a metal frame light switch and property is old with no RCD and there is some contact between Line conductor and earth or lets say metal work, in the situation how would the disconnection is achieved to prevent electric shock?

It gets thrown around a lot that 'the regs aren't retrospective' as if it's the 11th commandment but you need to consider the change of use due to technology too. You will see a lot of properties with only one socket in a room there just wasn't the same amount appliances back then and no electronics. If it's a old lady that still lives like it's the 70s it might still be safe...

In the situation you mention the metal frame would be earthed and the Zs would be low enough to disconnect within 0.4 secs. We only consider one fault so the external conductive part (which I assume isn't in contact with earth or bonded) couldn't become live without two simultaneous faults. A fault outside of the enclosure would be avoided by double insulated wiring which should be adequately protected against mechanical damage.

 

[timhoward]

Keep the questions coming.

If there is a metal frame light switch and property is old with no RCD and there is some contact between Line conductor and earth or lets say metal work, in the situation how would the disconnection is acheived to prevent electric shock?

This would cause very high current flow.
Nominal voltage (230) divided by resistance to earth (under 5.82 ohms if installed to regs and it’s a B6 breaker) gives fault current of 39.5 amps.
The graph for a B6 breaker shows this would trip instantaneously.

On a TT system it’s because the resistance to earth will be much higher and there won’t be enough fault current that a different method is needed (an up front RCD)

(Feel free to keep asking questions - but for basic theory John Ward made some very good YouTube videos.)

 

[King84]

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.

In TN as you guys explained ADS provides fault protection by disconnecting in 0.4s for final circuits and submain in 0.5s hence RCD is not required as long as we have additional protection in baths such as supplementary bonding or if the sockets are not in a place suspected of feeding outdoor then even without RCD older installations are safe as long as ADS conditions are met.

Sorry the above definitions were just for myself that I understood the concept from you guys properly lol.

Thanks alot everyone, you guys made this so much easier and I really appreciate you guys taking the time and effort to explain.
I wish the videos and books were interactive like you guys made the whole concept so much easy even someone like me understood this.
I tried asking elsewhere on another platform and guys were not helpful.
cheers to you All!

 

[Aaron b]

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.

 

[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.