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Automatic disconnection of supply

Discuss Automatic disconnection of supply in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

30mA RCDs are designed to trip when they detect earth leakage greater than 30mA.
Most will trip below this figure.
So if an RCD detects an earth fault current of 4600A, it will trip, and it will trip within 40ms.
An MCB will also trip if it detects a fault current of 4600A, though it is more likely to trip instantaneously or within 10ms.
Where RCDs are good, is that they will trip at lower currents which an MCB might not trip at, or may take seconds, minutes, hours or even days to trip.

So no, RCDs do not limit fault current, just as fuses and MCBs don’t limit fault current.
 
Explain the fault curve on a fuse chart if earth fault current doesn't grow with time. It shows how long it takes the fault to grow to a large enough magnitude to rupture the fuse. An RCD gets that curve chopped early. But by a means of imbalance measured via earth leakage
 
Last edited:
rich3009 said:
Well I would like to know the science behind how it saves your life if it doesn't limit the current your body receives
Click to expand...
Imagine you’re outside mowing your lawn with your hover mower.
The grass is damp, the cable is damp, you accidentally cut through the cable.
You make the mistake of picking up the damp live part of the cable.
You get a belt, you fall down and are unable to release the cable.
Because there’s no CPC in the cable and you’re outside, the fault path is via the general mass of the earth, which in your garden has a high resistance. So high in fact, that there is not enough current to cause the MCB to trip.
You manage to hang on to life for 20 mins, unfortunately it takes your partner 30 mins to wonder what you’re doing and come looking for you.
If there had been a working RCD, the power would have been cut off in 40ms, you’d have got a belt and your arm might ache for the rest of the day.
 
I was lucky once, or maybe I was sensible?
I borrowed a hover mower from a relative to mow my lawn.
I did not cut through the cable, but someone else had and made a botched repair.
I finished mowing, unplugged the cable near the mower and started winding the cable round my arm, walking back towards the socket in my out house.
The cable was damp and as I got near to the botched repair I got a belt.
Fortunately, the RCD socket I had installed in the out house tripped, though the one in the CU did not.

You’re not supposed to have RCDs in series, but I installed the socket before the CU was upgraded by the Council.
The difference between an RCD plug or socket and an RCD in a CU, is that every time the plug or socket is used, they are quite likely to be tested.
Whereas no one bothers testing the RCD in the CU.
Who wants the hassle of resetting the video, TV, stereo, cooker, etc.
 
rich3009 said:
Well I would like to know the science behind how it saves your life if it doesn't limit the current your body receives
Click to expand...
..Only any good with 230-250V AC. to earth
It limits the time the shock is received for (if it works !-Not sticking)
, and you restrict the current (unless covered on Copper mesh !)
So you don't get one of those "gargling moment's" !
 
spinlondon said:
30mA RCDs are designed to trip when they detect earth leakage greater than 30mA.
Most will trip below this figure.
So if an RCD detects an earth fault current of 4600A, it will trip, and it will trip within 40ms.
An MCB will also trip if it detects a fault current of 4600A, though it is more likely to trip instantaneously or within 10ms.
Where RCDs are good, is that they will trip at lower currents which an MCB might not trip at, or may take seconds, minutes, hours or even days to trip.

So no, RCDs do not limit fault current, just as fuses and MCBs don’t limit fault current.
Click to expand...
Rcds are advertised to operate at 30ma but they are actually manufactured to function at 27 to 28ma. Manufacturers hedging their bets on that one:D
 
rich3009 said:
  1. Preventing a current passing through the body of any person or any livestock
  2. Limiting the current which can pass through a body to a non-hazardous value
Click to expand...
An RCD functions to disconnect before most fatal events occur in accordance with the below graph, the disconnection time is such that the human body should be able to withstand the expected fault current flowing through the body for that duration without death, but not without pain and reaction, in most cases.
To prevent a current flowing or to limit the current to a non hazardous value would be achieved by non conducting locations, SELV or electrical separation. However the primary means of preventing a current is to make sure that conductors are not accessible to touch in the first place.
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