Massive RCD confusion (Australia)

[lechunnel]

So I'm replacing my house's switchboard (which is 40+ years old) and I'm required to install several residual current devices (aka ground fault circuit interrupters - GFCIs - in the US). My googling has given me a pretty good idea of how they work, but I'm confused about the circumstances in which they actually do anything, in terms of real life scenarios.

This is how I understand it: if an appliance plugged into the mains is live, and someone somehow comes into contact with the live circuit, the RCD will detect the current leakage, flick off, and turn the power off also, saving that person's life.

That much I get. But a mate of mine who has some electrical training pointed out that the RCD wouldn't trigger if the person was wearing rubber-soled shoes, since the person wouldn't be grounded. The end result would be the rubber-soled-shoe-wearing person would be electrocuted, since the power wouldn't turn off. So since 90% of shoes you wear are rubber-soled, doesn't that render the device a bit useless?

And this got me thinking more. Let's look at another scenario: a bunch of water on a bathroom or kitchen floor, that somehow has mains current running through it (say, the plug end of a live extension cord is sitting in it). This death trap would sit there until someone walked into it, in which case the RCD would flip, cutting off the current. Right? But not if that person was wearing rubber shoes (though I guess if that were the case, they'd be protected from the current anyway)? Or would the RCD flip as soon as the power came into contact with a grounded load that was sufficient enough to cause a significant current leakage?

As you can see I'm highly confused. Can someone explain in what circumstances an RCD will trip in practical terms? In particular, the two scenarios outlined in this post?

 

[Monkeh]

They wouldn't be electrocuted if they weren't grounded, because there'd be nowhere for the current to flow..

 

[chapeau]

Essentially

There are only two routes the electrons coming into your house can take out of your house and back to the transformer that pushed them to your house in the first place.

1) Through the neutral wire
2) Through the Earth

The RCD monitors the live and neutral wires. If live current <> neutral current it will trip. Another way of thinking about it is to say that earth current is greater than zero (or to be more exact, greater than the number of milliamps the RCD will trip at)

The only way live current <> neutral current is if electrons are moving out of the house via the earth.

Therefore any real world scenario where your body is electrically connected to both the live and the earth beneath your feet will cause the RCD to trip.

To give you further thought.... It will not trip if you are wearing rubber boots, you hold the live wire with your left hand, and the neutral wire with your right hand. You will get a possibly fatal shock, but the RCD will not trip. That is because live current = neutral current (and earth current = 0)

In your puddle of water on the floor scenario, the RCD would trip immediately the mains power hit the water and long before anybody walked into it. You should now be able to work out why.

 

[lechunnel]

Thank you both for your help!

Essentially
To give you further thought.... It will not trip if you are wearing rubber boots, you hold the live wire with your left hand, and the neutral wire with your right hand. You will get a possibly fatal shock, but the RCD will not trip. That is because live current = neutral current (and earth current = 0)

I see. And this would also be the case if, say, I came into contact with the power running through an appliance that was running, correct? Because that would essentially be the same as grabbing the live wire in one hand and the neutral in the other, right? The net result being current running through me.

And secondly, if the RCD did not trip (because current in live wire = current in neutral wire), would any circuit breaker/fuse do anything?

 

[lechunnel]

Apologies for the bump, but I'd love some clarification on my last questions if anyone had the answers!

 

[winston1]

If you grabbed both the live and neutral wires the RCD would not trip because the currents are equal. It's unlikely the circuit breaker would go either as you would over 10 amps through your body to trip it. You would be very uncomfortable.

Generally with a faulty appliance the fault current flows to earth which rapidly trips the RCD if fitted.

A RCD greatly improves safety and should be fitted but it can't cover every eventuality.

 

[TicklyT]

An RCD trips when a certain amount of current flows to earth instead of it's normal flow from live to neutral. The amount of current is set at a level that is a compromise between safety (preventing electrocution) and practicality(preventing nuisance tripping due to the natural 'leakyness' of some appliances - computers, for instance are naturally 'leaky')

The value often chosen is 30 milliamps. That is level is capable of delivering a shock, but is not usually fatal, and it is relatively immune to nuisance tripping. Other values may be more appropriate in other areas (eg bathrooms)

The 'live in one hand, and neutral in the other' scenario suggests either bad working practise, or a serious attempt at suicide. It will not trip an RCD, as they are not designed to detect that. They might trip if there is enough leakage current through the soles of your shoes to earth, but that's all.

They will (or should) trip nicely in the event of accidental contact with either live or neutral and earth, by, say, a faulty electric kettle in one hand, and a water tap in the other, because that is what they are designed to do.

Fuses or MCBs only provide over-current protection for wiring, not people, so they only break the circuit when there's enough current flowing to overheat the wiring, which is more than enough to kill a person.

About the only thing that can be relied upon is common sense and safe working practises to avoid the 'live in one hand, neutral in the other' scenario ever occurring in the first place.

 

[clandor]

circuit breakers are not there for personal protection, they are installed to protect the wiring and anything permanently connected to the circuit from short circuit or overload conditions,

switchboard replacements involve playing with mains and should only be done by someone with experience and currently holds an electrical practicing license, if this is the main switchboard of the house it will require an inspection as well. There is great potential to cause real dangers.

All information about how many and what rating of rcds to use can be found in as/nzs 3000:2010. Dont look at the older versions of 3000 as 2010 has revamped all rcd rules and is now in force.

Just as a side thought, if the cabling is 40 years old it may iether be rubber or have uninsulated earths, if so these cables have a tendency to create nuisance tripping.

 

[Monkeh]

have uninsulated earths, if so these cables have a tendency to create nuisance tripping.

Funny, we still use bare earths in new installations without such problems.

 

[clandor]

Funny, we still use bare earths in new installations without such problems.[/quote]

Ive seen it a ton of times in New Zealand, and we have the same cable types and cert standards as aus for cable imports.

NZ and Aus fazed out bare earths in the 70s or 80s, its more a problem with aged cables that have a breakdown of insulation due to being in ceiling with high temps, then anything else.

 

[Monkeh]

Ive seen it a ton of times in New Zealand, and we have the same cable types and cert standards as aus for cable imports.

NZ and Aus fazed out bare earths in the 70s or 80s, its more a problem with aged cables that have a breakdown of insulation due to being in ceiling with high temps, then anything else.

Nothing to do with the cable type, then, just the quality. Early PVC cables weren't exactly stellar.

 

[clandor]

cable type indicates age and likely hood of problems. This is why i said "have a tendency to create nuisance tripping."

 

[OwainDIYer]

Just as a side thought, if the cabling is 40 years old it may iether be rubber or have uninsulated earths, if so these cables have a tendency to create nuisance tripping.

An electrical textbook of about that vintage recommended bare earth wires as maximising the opportunity for any faults to run to earth and blow the fuse.

There shouldn't be any bare live wires floating about anyway, and the switches etc are often surrounded by a bare earthed metal back box so a bit of green/yellow won't make much difference to an otherwise safe, or faulty, installation.

 

[clandor]

An electrical textbook of about that vintage recommended bare earth wires as maximising the opportunity for any faults to run to earth and blow the fuse.

There shouldn't be any bare live wires floating about anyway, and the switches etc are often surrounded by a bare earthed metal back box so a bit of green/yellow won't make much difference to an otherwise safe, or faulty, installation.[/quote]

the earth wire is not considered a live wire, what i am talking about is a breakdown of insulation somewhere along the length of the cable (not at one of the ends). A breakdown can cause a high resistance fault between one of the live conductors (phase or neutral) and the bare earth. If the fault is high enough resistance there will be a slight leakage to earth. As there is current drawn on that circuit the current will split as the bulk will travel down the path of less resistance (ie return via neutral), as more current is drawn there is more leakage current returning via the earth, when fault current reaches 30mA the rcd will trip.

This is a common scenario with the old TPS cables that have bare earth conductors. I have seen it many times, and it can cause a nightmare to find the location of the fault or faults.