electtic shock - RCB did not trip

jkgray

Last night, as I reached under the shade to turn on a table lamp, I got an electric shock - because my partner had removed bulb to replace the one that had gone on her side of bed (!!!) and I put my finger into the 'socket'!

There was no burn mark on my finger and my partner who is nurse practitioner (and former A&E nurse) said I will be fine (as well as laughing a lot!).

Why did the new (about 1 wks ago) 17th edition consumer unit not trip?

Any thoughts?

ic

Is it a split consumer unit? I think (not being an expert) that the RCD only has to be fitted to the rings that can be used in wet zones or outside - generally this means the downstairs and kitchen must be covered, but the upstairs and light rings don't. The theory is that way the RCD protects anything plugged in on an extension lead and used in the garden, or anything at risk of getting splashed in the kitchen. Sockets upstairs are unlikely to have the same issue.

See this consumer unit as an example: http://www.screwfix.com/prods/99604/Electrical-Supplies/Consumer-Units/Volex-Consumer-Units/Volex-12-Way-Split-Load-Unit# - it states that of the 12 circuits it can support, up to 8 circuits are protected or 8 aren't.

TBH it seems dangerous to me not to have everything protected, perhaps by a dual RCD consumer unit (plenty are available).

keystone

jkgray wrote: »

Any thoughts?

Yes - don't do it again!

Cheers

Googlewhacker

keystone wrote: »

Yes - don't do it again!

Cheers

LOL sorry shouldn't laugh but it did make me.

Inactive

keystone wrote: »

Yes - don't do it again!

Cheers

Was that a " light bulb " moment?...:)

vaio

I’d expect your OH to be as sympathetic as she was (and medics are worse than most) but I’m appalled at the lack of sympathy you are getting in here.

Go look at the screwfix link given earlier, the extreme right end has the RCD and the mcbs adjacent to it are protected by it.

The extreme left is the main switch and the mcb next to it are not protected by the rcd. Compare it to yours and if the mcb that feeds your light is rcd protected then get your electrician back to sort it out as it seems likely something is wrong

keystone

vaio wrote: »

I’d expect your OH to be as sympathetic as she was (and medics are worse than most) but I’m appalled at the lack of sympathy you are getting in here.

OK - Ill be serious then.:D

If the circuit in question is RCD protected the RCD may well not have tripped in this instance but that doesn't mean anything is wrong. RCDs work by sensing a current imbalance, so if there's no current flowing there's no imbalance.

If you short circuit live to earth a current flows through the live but not through the neutral so the RCD trips. However, if you short neutral to earth the RCD doesn't trip as both are at the same potential and as a result no current flows.

If there is a load on the circuit there are equal currents in live and neutral conductors and so the RCD doesnt trip. There was no load on the circuit as the lamp had been removed. By putting his finger in the lampholder the OP provided a load (himself) but the current flowed live to neutral and not to earth (assuming the lamp is double insulated) so there was no imbalance - hence no trip.

Cheers

rygon

Are you sure they are RCDs, not MCBs?

Difference in looks are like this http://www.docep.wa.gov.au/EnergySafety/PDF/Misc/RCD_1zoom.jpg

MCBs protect the circuit, whereas RCDs protect the user

fluffpot

If your consumer unit was fitted within the last 2 years pretty much all circuits will be RCD protected. The main thing is that you're alive with no damage. the RCD will only trip if 30mA of current flows, so perhaps your shock was less than this... I don't think you'd have created a short circuit with your fingers, so that's why the circuit breaker didn't trip.

Fluff

DVardysShadow

keystone wrote: »

OK - Ill be serious then.:D

If the circuit in question is RCD protected the RCD may well not have tripped in this instance but that doesn't mean anything is wrong. RCDs work by sensing a current imbalance, so if there's no current flowing there's no imbalance.

If you short circuit live to earth a current flows through the live but not through the neutral so the RCD trips. However, if you short neutral to earth the RCD doesn't trip as both are at the same potential and as a result no current flows.

If there is a load on the circuit there are equal currents in live and neutral conductors and so the RCD doesnt trip. There was no load on the circuit as the lamp had been removed. By putting his finger in the lampholder the OP provided a load (himself) but the current flowed live to neutral and not to earth (assuming the lamp is double insulated) so there was no imbalance - hence no trip.

Cheers

Neutral and earth are rarely at the same potential - except at the connection of the system neutral to earth at the substation. The neutral will carry a return current - even on 3 phase systems this can be quite significant - leading to a voltage differential on the neutral between the substation and the consumer. Earth by contrast normally does not carry a current, so the voltage at the consumer is more nearly the neutral voltage as seen at the substation.

The difference in voltage is small [1's of volts] but enough to drive sufficient current to trip a sensitive RCD.

keystone

In the limit you are absolutely correct, of course, which is why:

1. With older non-double insulated metal light fittings a tingling sensation can sometimes be felt when touching it and;

2. Even with an MCB in the off position (MCBs being usually single pole) the circuit it protects is never truly dead and your meter can register a very small current t'wixt neutral and earth.

However, IIRC the pd between neutral and earth needs to be of the order of 50 volts to generate the 34mA current sufficient to trip an RCD so a very small pd of the order of 1 or 2 volts will not cause the RCD to trip.

Cheers