to earth or not to earth.

[dekachad]

but doesnt a rcd over ride a mcb? even the high resistance of a person.

 

[Paul.M]

[/LIST]chad;229101]I understand what a mcb and a rcd are for but still can't see the need for a cpc. As an earth fault in an appliance would not cause an overcurrent if no cpc was present, untill the appliance was touched, then the rcd would trip, over riding the mcb. Plus you would know the appliance you just touched and tripped the rcd is faulty.

If no cpc was present the appliance would be energized and be a hazard to life. If the appliance has a cpc connected via the 3 pin plug, into the washing machine and then connected to the outer metal body. Once a fault occurs it would have a path back to earth tripping the rcd or mcb without anybody being shocked.

 

[IQ Electrical]

An earth-free system in a domestic property would be almost impossible to implement due to extraneous conductive parts etc. introducing earth potential into the location.

The cpc has absolutely nothing to do with overcurrent protection.

Even using a TN supply system, you would be 'importing' an earth via the earthed neutral conductor which in the event of a neutral fault to an exposed conductive part, would earth the equipment.
Earth-free local equipotential bonding can be used but requires careful planning and supervision to ensure that the specialist requirements are adhered to.

 

[Paul.M]

but doesnt a rcd over ride a mcb? even the high resistance of a person.

They both do different jobs but with the same result of cutting off current for safety.

 

[IQ Electrical]

I hit the post button too early!

In the UK it is deemed that ADS (formerly EEBADS) is the safest, easiest and most economical way of providing fault protection.

It's purpose is to limit (during a fault) the magnitude and duration of the voltage between the exposed conductive parts of a circuit and other exposed conductive parts or extraneous conductive parts or 'true earth' so as to prevent danger.

To use the measure ADS, we need :

1 Basic protection of live parts.
2 Protective earthing
3 Protective bonding
4 Automatic disconnection in case of a fault

That is why we have a need for a CPC and bonding conductors.

 

[dekachad]

paul.m.............so are you saying a rcd needs an earth to work?, I thought any inbalance between neutral and live (i.e a few amps starting to flow thru a person) triggers the rcd.?

 

[dekachad]

IQ...... does that mean that if an appliance develops an earth fault and there is no cpc, then say a radiator 1m away, could become live.?

 

[IQ Electrical]

paul.m.............so are you saying a rcd needs an earth to work?, I thought any inbalance between neutral and live (i.e a few amps starting to flow thru a person) triggers the rcd.?

An RCD device provides 'additional protection' and BS7671 precludes the use of an RCD as the sole means of protection.

 

[dekachad]

rules and regs aside

 

[IQ Electrical]

Don't forget that an RCD will detect a line-cpc and a neutral-cpc fault so without a cpc there's not much point to the RCD.

A line-neutral fault will appear in effect, as a load and so the device will not operate.

 

[Ponty Massive]

A MCB is there purely to protect the cable. To prevent the circuit drawing too much current (in fault conditions or not). Why would you not put a 40A MCB on a 1.5 T+E cable? Because the cable would breakdown (over a sustained period) before the MCB would trip.

A RCD must have a path to earth before it will operate. Having a very low resistive earth in place will cause a very high current to flow in a fault condition and operate the RCD. If there is no earth in place and a fault occurs, then you have to become that path to earth before the RCD will trip.

As we only use an RCD as additional protection, if the RCD was to fail and there was no earth in place, when you put your hand on the washing machine, you become that earth, only this time the current has to reach the MCB rating (as aposed to 30mA) before it will trip. With the very low resistive earth in place and a failure of the RCD, the current should shoot through the roof and trip the MCB very quickly (hence our calculations for disconnection times and Ze)

 

[RogertheBodger]

"Rules and Regs aside" Look at it this way.

An RCD supplies a circuit with 1 socket outlet.

If you stick your finger in the live and you've got your insulated boots on, you won't get a shock because you are not earthed.

If you stick your other finger in the neutral you WILL get a shock (Big time ) because you have now become the load, but the RCD won't trip because the current passing through the L & N , and your body, is keeping the RCD in balance, You need to take one of your fingers out and touch the earth and then the RCD will trip immediatly because the resistance of your body is enough to cause an imbalance in the RCD of more than 30Ma.

If you can keep hold of the live and neutral long enough, and by now your hair is on fire, the MCB will trip because an overcurrent will be going through your body.

Earths are good!

 

[dekachad]

right, now we are getting somewhere. so a rcd will not work without a cpc? that is what i was taught at college but i believe now, it is an inbalance between neutral and live that triggers the rcd wich would occur in any earth fault or shock situation.

 

[tonys]

Hi all, new to the forum.
My question is, rules and regulation put aside......what are the pros and cons (if any) of earthing a house wiring system with all circuits protected by rcd's? i.e what would be the problems of a "no earth system" and what problems are created by earthing the circuits.? or is an earthing system just backup protection in case of a faulty rcd.?

Do you know of an installation with no Earth ?

 

[IQ Electrical]

"Rules and Regs aside" Look at it this way.

An RCD supplies a circuit with 1 socket outlet.

If you stick your finger in the live and you've got your insulated boots on, you won't get a shock because you are not earthed.

If you stick your other finger in the neutral you WILL get a shock (Big time ) because you have now become the load, but the RCD won't trip because the current passing through the L & N , and your body, is keeping the RCD in balance, You need to take one of your fingers out and touch the earth and then the RCD will trip immediatly because the resistance of your body is enough to cause an imbalance in the RCD of more than 30Ma.

If you can keep hold of the live and neutral long enough, and by now your hair is on fire, the MCB will trip because an overcurrent will be going through your body.

Earths are good!

It won't actually trip, the human body is normally between 1500 and 2000 Ohms so that would be asking a lot!