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# What trips an RCD????

Discuss What trips an RCD???? in the Electrical Forum area at ElectriciansForums.net

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#### wisesparky

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Hello guys, just a quick one, What trips an RCD? is it all about electric shock? if so and all limits in the test are meet, can i hold the line???????
Many times said - you need rcd protection, but what trips it????
Sorry but would love answers, thank you.....

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#### bazza2541

Imbalance in the circuit trips an rcd.
It basically measures the current going out on the live and compares that to the current coming back on the neutral, if there is an imbalance, caused by say a leakage to earth, then the rcd trips.

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#### wisesparky

Imbalance in the circuit trips an rcd.
It basically measures the current going out on the live and compares that to the current coming back on the neutral, if there is an imbalance, caused by say a leakage to earth, then the rcd trips.
When you say leakage to earth does this mean a human or the likes touching the line???
Thanks for getting back to me so quickly.

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#### bazza2541

Any leakage to earth,such as say the earth coming into contact with a live part of the equipment, but a human would do the job.

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#### desertbootz

Both Live conductors (Live and Neutral) are tapped off to give an equal number of windings around opposite sides of a ferrite toroid (a ring of material that becomes magnetic only when another magnetic field is present) As the number of turns are equal and the current direction is opposite at any given moment (AC is flipping between +ve and -ve remember) their influence on the magnetic field induced on the toroid is balanced out. Normally (there are exceptions) whatever goes in to a circuit, in terms of current, comes out again so as far as the toroid is concerned, under normal circumstances, everything balances. If a pre defined amount of that current goes elsewhere up someones arm for instance when they get strapped across the mains, there is now an imbalance in the induced fields on the toroid. That imbalanced magnetic field creates a small current in the toroid which is detetcted and used to actuate the isolation built in to the RCD.

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#### wisesparky

Thanks guys, and thanks for explaining in great detail, very, very much appreciated.
Thanks again
Justin

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#### Guest123

Both Live conductors (Live and Neutral) are tapped off to give an equal number of windings around opposite sides of a ferrite toroid (a ring of material that becomes magnetic only when another magnetic field is present) As the number of turns are equal and the current direction is opposite at any given moment (AC is flipping between +ve and -ve remember) their influence on the magnetic field induced on the toroid is balanced out. Normally (there are exceptions) whatever goes in to a circuit, in terms of current, comes out again so as far as the toroid is concerned, under normal circumstances, everything balances. If a pre defined amount of that current goes elsewhere up someones arm for instance when they get strapped across the mains, there is now an imbalance in the induced fields on the toroid. That imbalanced magnetic field creates a small current in the toroid which is detetcted and used to actuate the isolation built in to the RCD.

#### mrloy99

##### -
Imbalance in the circuit trips an rcd.
It basically measures the current going out on the live and compares that to the current coming back on the neutral, if there is an imbalance, caused by say a leakage to earth, then the rcd trips.
Hi Barry what about 3 phase rcds? whats the deal with them?

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#### snowiespark

same, just they have coils that measure all phases... they more complex but same theory

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#### pushrod

Hello guys, just a quick one, What trips an RCD? is it all about electric shock? if so and all limits in the test are meet, can i hold the line????.....
just a quick point in addition to what has already been mentioned - RCDs are not really there to prevent electric shock ( although they can reduce the risk) there main job is to protect the installation.

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#### Techymike

Quote:
Originally Posted by wisesparky
Hello guys, just a quick one, What trips an RCD? is it all about electric shock? if so and all limits in the test are meet, can i hold the line????.....

just a quick point in addition to what has already been mentioned - RCDs are not really there to prevent electric shock ( although they can reduce the risk) there main job is to protect the installation.
__________________

Disagree with that one - RCD's are not designed to protect the installation. If there was a short circuit in the load causing high currents to flow which could damage the installation, as long as there is no imbalance between phase and nuetral, the RCD would not operate. It is the job of the circuit breaker to protect the installation, and the job of the RCD to provide additional shock protection.

Cheers,

Mike

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#### wot

RCDs are not really there to prevent electric shock ( although they can reduce the risk) there main job is to protect the installation.
Are you sure ? .... RCD's can provide additional protection in the event of a failure of basic protection against electric shock. See 415.1.1
I would agree that the fuses and circuit breakers are there to protect the installation but not the RCD, which does also have specific fire prevention uses in some special areas.
What do you think ?

Regards, wot.

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#### pushrod

If there was a short circuit in the load causing high currents to flow which could damage the installation, as long as there is no imbalance between phase and nuetral, the RCD would not operate. It is the job of the circuit breaker to protect the installation, and the job of the RCD to provide additional shock protection.

Cheers,

Mike
You are quite correct in that a short between live and neutral will not trip an RCD, but then an RCD is an earth fault device and not designed for that or as sole protection, although it's importance increases in TT systems where Zs can be become too high.
I maybe wrong but i am sure i have read or been told that the prime function of an RCD is installation protection, with the very real benefit in most cases of shock protection. If its prime function was shock protection why would you have 100mA examples that would allow a lethal shock?

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#### Deleted member 9648

.
I maybe wrong but i am sure i have read or been told that the prime function of an RCD is installation protection, with the very real benefit in most cases of shock protection. If its prime function was shock protection why would you have 100mA examples that would allow a lethal shock?

It all depends on the RCD in use. 100ma - 300ma and time delayed RCD's are intended to protect the installation, usually in a TT system where Zs is too high for the required disconnection time to be met by fuses and mcb's. The prime function of a 30ma RCD in most installations, including TT is to provide supplementary protection against electric shock where the risk is considered increased....ie sockets, outside,bathrooms etc....After all 30ma RCD's are widely used in systems where the required disconnection time is met by the fuse/mcb alone

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#### wot

Yes, that 's all correct isn't it. So now I suppose we could say that the RCD is a multi role device depending on how it is a used;

To help disconnection times when Z's are too high.
To help with basic protection against electric shock.
To help prevent fires (farms 300mA).
Where discrimination is needed between RCD's, fit a higher rating.
Installations with high earth leakage, fit a higher rating.

Does that summarise where we are ?

Wot.

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