MET and ADS

[askawaysparky]

Ok so I'm finishing off some questions for a mock exam and its come up with:

"Explain why an exposed-conductive part MUST be connected to the MET to facilitate ADS?"

But my understanding is that anything that automatically disconnects the supply is connected to the Line(MCB) or the Neutral Bar(RCD), not the main earth terminal, Can someone explain to me how this works and what I'm missing?

am I mistaking the MET for the Earth bar when its something else? I'm not sure

EDIT: I think I have clicked onto it?, Is it that the current would flow through the exposed part and to the MET which would trigger the RCD because it would detect that current wasn't following residually anymore?

 

[dodger421]

Ok so I'm finishing off some questions for a mock exam and its come up with:

"Explain why an exposed-conductive part MUST be connected to the MET to facilitate ADS?"

But my understanding is that anything that automatically disconnects the supply is connected to the Line(MCB) or the Neutral Bar(RCD), not the main earth terminal, Can someone explain to me how this works and what I'm missing?

am I mistaking the MET for the Earth bar when its something else? I'm not sure

Think about the Earth Fault Loop (draw it out if you need to) and what would happen if a live conductor came into contact with an exposed-conductive part that wasn’t connected to the MET. Where would the fault current go, what would cause ADS to operate?

 

[timhoward]

Maybe swap terms for tangible things.
And forget RCD's completely, assume there aren't any.

Imagine you had a metal class 1 desk lamp plugged in.
Why does the body need connecting to the MET?

 

[askawaysparky]

Think about the Earth Fault Loop (draw it out if you need to) and what would happen if a live conductor came into contact with an exposed-conductive part that wasn’t connected to the MET. Where would the fault current go, what would cause ADS to operate?

Im guessing it wouldn't operate?

 

[askawaysparky]

Maybe swap terms for tangible things.
And forget RCD's completely, assume there aren't any.

Imagine you had a metal class 1 desk lamp plugged in.
Why does the body need connecting to the MET?

to stop you from getting a belt if the line comes into contact with it

 

[timhoward]

to stop you from getting a belt if the line comes into contact with it

Right. Or in other words - to provide a return path for the current under fault conditions.

 

[dodger421]

to stop you from getting a belt if the line comes into contact with it

Yes, by ADS. You need to think about how ADS devices work to clear a fault and what is meant by “Earth fault loop”.

 

[askawaysparky]

Right. Or in other words - to provide a return path for the current under fault conditions.

but I take it the RCD wouldn't trip if it touched the lamps body and it wasn't earthed?

 

[timhoward]

but I take it the RCD wouldn't trip if it touched the lamps body and it wasn't earthed?

You are correct about that. But.... It can help to think that you have three levels of protection, with the following simplified definitions:
Basic Protection (make sure you can't touch anything live)
Fault Protection (protection for overload, short circuit, including line conductor touching exposed conductive part)
Additional Protection (RCDs to turn it off damn quick if current that goes out doesn't come back)

A question about ADS will generally be about Fault protection as that always has to be there. RCD's don't always (though increasingly are).
To add it isn't quite that simple in all circumstances (e.g. TT installations) but it's not a bad starting point IMHO.

 

[askawaysparky]

You are correct about that. But.... It can help to think that you have three levels of protection, with the following simplified definitions:
Basic Protection (make sure you can't touch anything live)
Fault Protection (protection for overload, short circuit, including line conductor touching exposed conductive part)
Additional Protection (RCDs to turn it off damn quick if current that goes out doesn't come back)

A question about ADS will generally be about Fault protection as that always has to be there. RCD's don't always (though increasingly are).
To add it isn't quite that simple in all circumstances (e.g. TT installations) but it's not a bad starting point IMHO.

wow ok brilliant thanks very much, so I will answer this with:

"Exposed conductive parts must be connected to the MET to provide a return path for current under fault conditions, without this there will be no ADS/Fault protection"

 

[timhoward]

"Exposed conductive parts must be connected to the MET to provide a return path for current under fault conditions, without this there will be no ADS/Fault protection"

That's certainly correct and not a bad answer. The final icing on cake would be to say that enough current needs to flow to operate the over-current protective device within the required disconnection time.
So the key concept is knowing how much current (or what the max Zs will be) will make the MCB trip or Fuse blow within the required time.

 

[askawaysparky]

That's certainly correct and not a bad answer. The final icing on cake would be to say that enough current needs to flow to operate the over-current protective device within the required disconnection time.
So the key concept is knowing how much current (or what the max Zs will be) will make the MCB trip or Fuse blow within the required time.

again brilliant thanks