Should the MCB have tripped?

[TheCodboy]

Hi all, looking for some input from you guys. Had a call out a few days ago where someone has has a shock from brass light switch in a domestic property. When i got there, a decorator told me that he had the switch loose from the wall to strip the wall paper. He's used a steamer, so it points to moisture getting into the switch. The woman who lives there then went to use the switch later and got a shock. Apparently quite painful. The lighting circuit is covered by a 6a mcb on the non-rcd side of the board. When i got there, the switch must have dried out and was functioning fine. I did a ZS at the switch (my megger 1741+ allows a 2 lead test) and i got a reading of 0.69. The CPC was connected to the earth terminal on the switch. So, should the MCB not have tripped?

 

[westward10]

Onĺy if sufficient fault current flows.

 

[TheCodboy]

Onĺy if sufficient fault current flows.

Which was my thoughts, but the woman is saying that it gave her quite a belt. Dizzy, sore arm etc. Possibly a bit of exaggeration on her part!

 

[westward10]

She may well have but her body resistance and the damp will have a high resistance which would have restricted the current flow.

 

[telectrix]

MCB would not trip.a RCD may have done, if fitted. bear in mind that a 6A MCB requires approx. 30A fir instantnneous trip. shocking,innit?.

 

[TheCodboy]

She may well have but her body resistance and the damp will have a high resistance which would have restricted the current flow.

I always think back to when i was a young'n and i was getting a tingle of a machine we had in for service. No one believed me as i was the only one, till the gaffer tested it and low and behold.... Fault!

 

[TheCodboy]

MCB would not trip.a RCD may have done, if fitted. bear in mind that a 6A MCB requires approx. 30A fir instantnneous trip. shocking,innit?.

Quite literally yes!!

 

[TheCodboy]

She may well have but her body resistance and the damp will have a high resistance which would have restricted the current flow.

I asked if her hands were damp etc.... The decorator was looking at us rather sheepishly.

 

[DPG]

Which was my thoughts, but the woman is saying that it gave her quite a belt. Dizzy, sore arm etc. Possibly a bit of exaggeration on her part!

If enough current to trip a 6A MCB had passed through her then she would have a lot more problems than a sore arm!!

 

[Teddy Lop]

If enough current to trip a 6A MCB had passed through her then she would have a lot more problems than a sore arm!!

Friggin not 'alf, you'd want a priest and a dustpan not a spark! OP do you understand the difference between MCB and RCD?

 

[TheCodboy]

Friggin not 'alf, you'd want a priest and a dustpan not a spark! OP do you understand the difference between MCB and RCD?

Yeah, to be honest with the switch being wet I thought that the MCB would have popped that's all.

 

[Roy1010]

If the metal of the switch has a good earth it cannot become live at all. Maybe it was the floor that was live or was she touching something else as well at the time? An electrical appliance in her hand? (bet it was static)

 

[Teddy Lop]

If the metal of the switch has a good earth it cannot become live at all. Maybe it was the floor that was live or was she touching something else as well at the time? An electrical appliance in her hand? (bet it was static)

There's normally an insulating plastic strip around the rocker (the cap itself if metallic will not be earthed) and it's the crack between this and the rocker cap through which the live parts may be only a few mm deep, and thus the damp may track a contact path. In fact being an earthed metal switch may have made it worse by neatly completing the circuit!

I've had a few clients recent times complain of being zapped when operating switches, only for the subsequent discussion to reveal they "might have had" wet paws, usually millennials! What happened to teaching of this to kids, along with looking before crossing the road! And you think you're responsible enough to have sockets in bathrooms, eh?

 

[LastManOnline]

I asked if her hands were damp etc.... The decorator was looking at us rather sheepishly.

Even if her hands were, nt damp she could still get a bad rap. Water or damp acts as a resistor. As with any resistor the voltage drops across it. The shock felt will depend on the point of contact with the resistor.