Emergency Stop implementation

Andrew Mawson · 2009-03-14T21:54:36+00:00

I'm planning a scheme for emergency stop buttons in the workshop andam considering using the existing RCDs as the switching element. Ihave a three phase board equipped with a 100mA RCD and a split singlephase board with the ring mains on a 30mA RCD. By having emergencystop buttons with two n/o contacts, each with a suitable resistorswitching a phase to ground I can introduce a momentary fault currentwhich trips the appropriate RCD but leaves the lighting circuits withpower. . Very handy having a button by the exit to ensure everythingis off when leaving.Seems very simple, and I've proved it works, can anyone see anydrawbacks to this scheme?AWEM

Mar 14, 2009 25 Replies

Rate this thread:

Andy Dingley 17 years ago

Most RCDs don't have a convenient earth terminated on them, so it's easier to do it by unbalancing the phase & neutral as they run through the current transformer. The detector circuit itself doesn't care how you do it.

Where you have a DC-sensing fault detection circuit (most RCBOs) it's not possible to do this simple imbalance, so they use the alternative of providing a simulated fault current to earth. Hence the need for a low-current earth connection on this type, usually as a small flying lead.

I'm not up to date on the legislative state of things, nor on the requirements between RCD / RCBO and AC / DC sensing. Pointers would be welcome.

Vote

Harry Bloomfield 17 years ago

Al wrote on 15/03/2009 :

Turning a motor or light on puts a resistor of sorts across the supply, as does pressing the RCD test button - the difference is?

On that we can agree.

Vote

Dave Osborne 17 years ago

The difference is a "resistor of sorts" between live and earth (RCD test button) as opposed to between live and neutral (normal load).

Vote

John Rumm 17 years ago

That sounds unlikely. Do you have a reference?

Vote

Dave Osborne 17 years ago

My bad, that should read 'The difference is a "resistor of sorts" between live and earth (deliberate remote tripping of an RCD)...'

Vote

Andy Dingley 17 years ago

The difference is that both connections to the load pass through the RCD's sensing current transformer (in a direction such that one cancels out the other and the nett current is zero), whilst the test load is only passed once through the sensor and so it represents an unbalanced current.

Current operated RCDs don't detect the "leakage current" per se (that would be difficult from a remote location), instead they detect an imbalance in Live and Neutral currents, then assume that any difference is a leakage to earth.

In fairness to Al, he's also talking against the variant where the test current is fed to earth, rather than this unbalanced current. My own sloppy wording might have sent the thread in that direction.

Vote

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required