Does this mean my rcd in my consumer unit could be faulty ?

[shaky1105]

We've had plasterers in today and during the rain, he decided to make another mix !! His mixer was soaked and as he switched it on, it gave him a "bit" of a shock. However, this didn't trip the rcd in my consumer unit.

Strangely though, a couple of weeks ago we were using a four-way trailing lead and a bit of residue out of the bottom of a coffee mug got spilt onto it and the rcd tripped with no drama ????

Would this be worth getting checked out or are there situations where an rcd wouldn't trip even though a slight shock could be felt ?

Thanks

 

[securespark]

All depends on the current flow and the rating of the RCD.

Also, the coffee may have been a better conductor than the water.

Difficult to say.

 

[ban-all-sheds]

Also, the coffee may have been a better conductor than the water.

Difficult to say.

Maybe you could work it out using the Maxwell House Equations....

 

[ericmark]

A RCD does stop you being electrocuted in most cases but not from getting a shock. The device needs between 15 and 30ma difference between line and neutral to trip so contact between line and neutral will not trip it at all only contact between earth and either line or neutral will cause it to trip.

It also can take up to 40ms for it to work.

Also most builders use reduced low voltage rather than low voltage and the transformer used isolates the line and neutral so it will not trip your RCD how ever much current flows to earth.

Although the test button is only really to test the mechanics of RCD to ensure it does not stick and a special meter should be used to test it pushing the button and it tripping normally means there is nothing wrong with it.

 

[Risteard]

Reduced Low Voltage is not isolated from earth. It is centre-tapped with 55V to earth and 110V between lines (there is no neutral conductor).

 

[shaky1105]

Maybe you could work it out using the Maxwell House Equations....

Also most builders use reduced low voltage rather than low voltage and the transformer used isolates the line and neutral so it will not trip your RCD how ever much current flows to earth.

The mixer they were using was 240V NOT 110V. Still, I suppose it may be worth getting someone in to do a test once I've had the building work completed.

Thanks for all the replies.

 

[AndyPRK]

And the RCD didn't trip after the shock I assume....

 

[shaky1105]

And the RCD didn't trip after the shock I assume....

Nope - see below....

as he switched it on, it gave him a "bit" of a shock. However, this didn't trip the rcd in my consumer unit.

That's why I was a bit worried. If he's daft enough to use a soaking wet mixer, that's his problem but I'm concerned about a safety device in my home that might not be working correctly.

Would a PIR be a wise move, or are these pretty expensive ? The wiring is only about 7 or 8 years old AFAIK

 

[ericmark]

Reduced Low Voltage is not isolated from earth. It is centre-tapped with 55V to earth and 110V between lines (there is no neutral conductor).

Although the earth is not isolated the line 1 and line 2 are so an earth fault would not trip a RCD.

However it was not 110v so that's not a consideration. However with the motors of the mixer there could have been some transformer action meaning the RCD would not be directly supplying the current which leaked to earth through the builders body.

To test the RCD does not take long although it does need a special meter. I can't see an electrician charging much to test the RCD although a full PIR would be very different.

 

[mfarrow]

I got a zap recently from a neutral conductor on a socket circuit from the RCD protected side of the board at home. The RCD was off at the time.

Turns out the person who wired this...

... into the lighting circuit (the lighting circuit is on the non-RCD side of the board) had wired the timer neutral into the RCD protected side.

Obviously there was an inbalance, but it was only 10mA so not enough to trip the RCD but enough for a good belt.

 

[plugwash]

Reduced Low Voltage is not isolated from earth. It is centre-tapped with 55V to earth and 110V between lines (there is no neutral conductor).

Indeed.

However a RCD does not detect current flowing to/from earth it detects an imbalance between currents in the current carrying conductors.

A fault in something connected to the output side of a site transformer cannot cause an imbalance between currents in the live and neutral of the input side of the site transformer. Therefore it cannot trip a RCD located before the site transformer (at least not directly).

You can of course have a RCD on the output of a site transformer but i've never seen such a setup in practice

 

[pjcomp]

Plasterer got a bit of a shock off his mixer but it didn't trip the RCD. Lot of drills (if not all) are Class II - double insulated, no CPC. If it was a drill with a paddle in it, or even a purpose-made mixer which is just the same thing, it probably was Class II and the fault is with the machine's insulation breaking down at some point.

PJ

 

[TicklyT]

A couple of considerations maybe in the 'as he turned the machine off' bit.

Perhaps the plasterer got zapped by back EMF from the motor after the switch contacts were opened, isolating it from the house electrics, but before the motor stopped turning.

Or, if a cap start motor, the starting capacitor discharging as the centrifugal switch reconnected it to the isolated circuit as the motor slowed.

 

[AndyPRK]

Your probably correct. It was back EMF. Which is only an annoyance.


If there were a tranformer involved, then your unlikely to get a shock anyway, as it isolates it from the mains. (unless earth is connected to the centre tap)