Why do we still need protective earth with an RCCB/RCBO?

Question

If most of your devices at home are already double-insulated, does the protective earth still serve a purpose when you have an RCCB/RCBO installed in your panel board?

I'm asking this since I live in a developing country wherein most residential homes don't even have a wire for protective earth distributed around all rooms. To compensate for this, most appliances/devices sold here are double-insulated. It's a pretty common practice for citizens here to, as much as possible, avoid buying devices which has a metal casing.

Now from what I've read, an RCCB and an RCBO prevents households from shocks by actively monitoring the current passing through the live and neutral wire with an electromagnetic coil. If that's the case, doesn't that essentially outmode the protective earth pin?

Answer 1

RCBOs/RCCDs are not sufficient for human safety protection, and that was never their purpose.

If you look, they have a 30mA threshold. This is a threshold the Americans call GFPE, or "Ground Fault Protection of Equipment". Its purpose is to detect hot-ground or neutral-ground arcing faults which might make excessive heat and start a fire. For this detection to work, Protective Earth (ground) must be wired.

The 30mA threshold is not useless for human safety protection, clearly stopping shocks at 30mA is an improvement on not stopping them at all. However if you want to make a good showing of human-safety protection, you really need to be at a 5mA threshold, which is exactly what North America does on GFCI protection, required in bathrooms, kitchens and other high-shock-risk locations. Note that America applies this locally, on a "per branch circuit" basis, or even on a "per receptacle" basis.

"But my whole house has ONE device. Can't we just turn a knob and sharpen that to 5mA detection?" You could. The problem is, normal wiring in the house has a tiny amount of leakage of its own. And when you get a whole house, that leakage can exceed 5mA from time to time. This will cause "nuisance trips" often enough to render the electric service practically useless.

Protective Earth is very helpful for RCD, GFPE and GFCI to detect faults early before a shock event ever happens.

If you are boxed into a choice between RCD or earthing, RCD will result in somewhat more human safety, but they really do work better together.

It's also notable that equipment spike and ESD protection depends on proper earthing to work properly.

Answer 2

The RCCBs and RCBOs common in Europe are set to trip at no more than 30mA. 30mA probably won't kill you if the power is turned off quick enough. But lower currents can still give you a nasty shock, which could cause an accident. There really isn't any safety margin in a 30mA breaker.

Also, they are complex electromechanical things. Sometimes they fail, and don't trip even on currents well in excess of 30mA.

Adding in an Earth connection provides a second, and completely independent, safety feature. If the live wire comes adrift and touches the case of an earthed appliance, a large current flows, and the fuse or breaker trips. Appliances relied on this "automatic disconnection of supply" long before resudual current devices were available. Note that on supplies that use "TT" earthing, where the only Earth connection is via an earth rod or similar, you can't rely on a fuse or breaker, and a correctly functioning residual current breaker is essential.