Drill Damaged Cable

[securespark]

Hi, drilled a hole in the wall and caught the live only of a flex with the drill bit of a Class II SDS drill.
Would that cause an imbalance detectable by an RCD?

 

[JohnW2]

Hi, drilled a hole in the wall and caught the live only of a flex with the drill bit of a Class II SDS drill. Would that cause an imbalance detectable by an RCD?

It shouldn't, since the chuck, hence bit, of a Class II drill would/should not be connected to earth.

Kind Regards, John

 

[bernardgreen]

It could do. The potential of the metal work of the drill will be some where between the potentials on the Neutral and the Live by capacitive coupling. Depending on the impedamce of the capacitive coupling there could be enough current flowing from the Live wire through the capacitive coupling in the drill to Neutral

 

[JohnW2]

It could do. The potential of the metal work of the drill will be some where between the potentials on the Neutral and the Live by capacitive coupling. Depending on the impedamce of the capacitive coupling there could be enough current flowing from the Live wire through the capacitive coupling in the drill to Neutral

I would think it incredibly unlikely that there would be enough capacitive coupling to trip an RCD. By my calculation, to get 30mA from 230V at 50Hz would require a capacitance of about 0.4μF - which I would have thought was orders of magnitude greater than one could expect from 'stray coupling'.

Kind Regards, John

 

[securespark]

I thought the same as you, John, but am failing to explain it!

 

[JohnW2]

I thought the same as you, John, but am failing to explain it!

Are you saying that it did trip an RCD?

If so, are you absolutely certain that the drill bit didn't come in contact with the CPC of the cable or anything else earthed, as well as the L (even if only very briefly)?

If the drill has a 2-core cable, then there is obviously no possibility that the drill bit is earthed via a CPC.

Kind Regards, John

 

[securespark]

Yes, it did and having deconstructed the cable, the only copper exposed was a small section of sheath and insulation to the line conductor. The neutral and cpc remain undamaged.

 

[JohnW2]

Yes, it did and having deconstructed the cable, the only copper exposed was a small section of sheath and insulation to the line conductor. The neutral and cpc remain undamaged.

Intriguing.

You didn't say whether the drill has a two core cable. If it did, then I really don't see any realistic alternative to the drill bit somehow having briefly touched the CPC (or something else earthed) as well as the L - since, as I said, I really can't believe that there would/could be enough capacitive coupling to be the cause of the trip.

What were you drilling through before you hit the cable?

Kind Regards, John

 

[bernardgreen]

to get 30mA from 230V at 50Hz

to get a continuous 30mA from 230V at 50Hz sine wave

applying a source of voltage to a capacitor is the same as putting a short circuit across the source until the capacitor has become charged to the same voltage as the supply.

Also with a drill there is the effect of the motor and it's magnetic components creating #, for want of a better word, eddy currents and voltages in much of the metal in the drill and gear box.

 

[securespark]

It is a Class II Bosch SDS drill with two core flex. I was drilling a hole in a wall and the bit skidded and grazed a flex.

Maybe bernard has something?

 

[JohnW2]

to get a continuous 30mA from 230V at 50Hz sine wave .... applying a source of voltage to a capacitor is the same as putting a short circuit across the source until the capacitor has become charged to the same voltage as the supply.

True, but it has, at most, half a cycle in which to do that 'charging' (realistically more like quarter of a cycle), during much of which period the capacitor/capacitance will be sufficiently charged to appreciably limit the current - and the average (RMS) current over any number of whole half-cycles will be that which one calculates from the reactance. What I don't know is the duration of a current >30mA necessary to trip an RCD.

In any event, since I can offer no better explanation for what secure experienced, I have to admit that what you are suggesting could have been the explanation.

The thing I find most difficult to understand about any such proposed mechanism is that I can't really see why the casing of a drill should/could have appreciably more capacitive couple to the neutral than to the line.

Kind Regards, John

 

[JohnW2]

It is a Class II Bosch SDS drill with two core flex. I was drilling a hole in a wall and the bit skidded and grazed a flex. Maybe bernard has something?

Was the drill plugged into a circuit which was protected by a different RCD from that protecting the cable you damaged (and, if so, which one tripped)?

As far as I can make out, bernard's proposed mechanism could not work if both the drill and the cable you hit were protected by the same RCD, since the postulated current through 'capacitive coupling' would then merely be between L & N of the same RCD - hence would not result in any current imbalance.

Kind Regards, John

 

[Blueflash]

I hope that cable was in permitted zone ? The RCD may have tripped because you wet you're pants !

Dave

 

[JohnW2]

I hope that cable was in permitted zone ? The RCD may have tripped because you wet you're pants !

However, I don't think it was a buried cable ...

I was drilling a hole in a wall and the bit skidded and grazed a flex.

Kind Regards, John

 

[securespark]

Was the drill plugged into a circuit which was protected by a different RCD from that protecting the cable you damaged?

RCD Incomer.

It was a flex clipped to an ash block wall.