This thread titled "to earth or not to earth." is posted in the under the Electrical Wiring, Theories and Regulations on Electricians Forums.
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.?
Before people give a detailed answer I must ask...are you a qualified sparky?
I am qualified but not much experience as its not my main vocation. Its a question I have pondered way before I qualified as I have seen many houses with no earth, abroad obviously.
Well I'll start by saying that RCD is NOT considered as main protection, it is additional protection.
A main earth is needed because this provides the earth fault path which is connected back to the middle winding of suppliers transformer (the star delta point. )
Try and find a diagram detailing the earth fault path, its a great example why one is used. If you want I can explain it out, but it may not make as much sense without the diagram.
We rely on the Mass of earth in some systems as our earth, others the suppliers earth.
Fuses/mcb's are different to rcd's. Mcb protects the circuit/cable from overload and rcd protects against electric shock.
The rcd monitors the current flow in the line and neutral conductors and operates if it detects an imbalance of 30mA.
A mcb in the case of a fault to earth will increase current beyond the fuse rating and trip/blow the device.
Some countries run in a dc current therefore no earth is needed. In the UK we need both rcd and mcb for two types of protection.
I remember doing this in college and I have the diagram in front of me. But it doesnt explain "why" we need an earth. I understand that an earth will protect against a faulty appliance but if there is no earth and the circuit is protected by a rcd, this will trip as soon as you touch the faulty appliance. Also, if I knock a nail thru a live wire, the rcd will protect me but an earth system will not.
Thanks paul.m...... so one con for the earth system is to protect the cable from overcurrent via a mcb. If an appliance develops an earth fault but there is no earth, would this still overlaod the cable?
Sorry, that would be a pro.
ok say if you were in an eastern european country where there is no or little earthing in place, but the premises had all modern consumer units etc with mcb/rcd but no earth, say you accidently cut through a cable would that create enough imbalance to trip the rcd?
Yes. Although no earth wire (or cpc) is present, it can still be earthed by direct contact from a person. Eg, a washing machine with no earth/cpc conductor develops a fault, the live wire inside touching the metal casing would not trip a mcb as it would now be part of the circuit with no way to get to earth until someone comes along and touches it and grounds it via your body. The earth will take as much current as the resistance can carry until the flow superseeds the rating of the mcb.
A rcd monitors the out going current on the live/line wire and the return current if the neutral and if the live and neutral go beyond 30mA (30/1000 of an A) it will operate and trip.
cpc is designed for disconnection time aswell, adibiatic equation,,
i'm right aint i.
So, really, the rcd would protect the cable from overcurrent, as it would trip out as soon as the appliance is touched. thus, no need for an earth?
Thanks paul.m...... so one pro for the earth system is to protect the cable from overcurrent via a mcb. If an appliance develops an earth fault but there is no earth, would this still overlaod the cable?
The earth/cpc is to allow a path to earth faster than a person. This in turn will request a much higher current than the rating of the mcb. Eg you cut a twin and earth 1.5 rated at 6A, current then flows from the live to the cpc back to earth in milli seconds. Because of the connection to the cpc the earth can take a much higher load than 6A and trips the mcb. If there wasn't a cpc it would travel via the neutral with the same effect. Another way a mcb trips is for it to be overloaded. Multiple loads on a circuit higher than the rating of the cable Thers why we rate the mcb at the same current capacity or lower than the cable.
In a rcd circuit continuously compares the current in live and neutral. If the circuit is healthy the two will be equal until a live to cpc occurs and when 30mA leaks to earth via the cpc or a person the rcd will trip.
Hope that gives you a better description of the two. Both are needed and a cpc/earth is also vital.
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.
lower the resistance, high the current , and we want to achieve large amount of current to trip mcb faster say 5 times the rated breaker for instantaneous 0.4 secs
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