Mains electricity - shocks

[PhillyDee]

Heh, I have 2 laser cutters in my business, with 80W tubes. The trigger voltage is around 25Kv at 20mA. And that's the power just to strike the arc in the water cooled tube. Plenty of cases of the tube electrifying the coolant water due to a leak.

Luckily, my machines are fully CE compliant, RCD's built into the cabinet. Unlike a lot of imported eBay lasers! I would hate to get a shock from those buggers!

My unit has 3 Phase, but everything I use is just standard 240V.

 

[Mynight]

Iirc it's to do with electromagnets essentially both live and neutral are wrapped round a copper coil creating a small magnetic field. When operating normally these fields cancel each other out however if there's a fault in one of the lines then the field is no longer cancelled and creates an imbalance.

There's a third coil (search coil iirc) upon the field no longer cancelling each other out it induces a current in the search coil which activates the relay cutting out the circuit.

 

[john_s]

Doesn't an RCD effectively compare the current in the phase and neutral conductors and trips if there's a difference in excess of it's rated value?

Ie, if in a fault situation (eg someone has touched the phase conductor) current is flowing to earth, then there will be a higher current in the phase than neutral, so the RCD trips.

If someone has hold of the phase and neutral conductors, there may well not be enough difference in the phase and neutral currents to trip the RCD, so the shock continues...

 

[Sliver]

What I can't get my head around is how the RCD can differentiate between current flow to neutral and current flow to earth when they are ultimately combined

RCD's work on the current balance principle, using Kirschoff's First or Node Law. That is the algebraic sum of all the electric currents meeting at any point in a circuit must be zero, otherwise the circuit will be broken.

In a RCD the incoming supply, phase and neutral, passes through an iron core or toroid, which acts as the primary winding of a current transformer. A secondary winding around the toroid is connected to the trip mechanism.

Under normal conditions the phase and neutral currents are equal and opposite so no flux is induced in the toroid and hence no current flows in the secondary winding.

If an insulation fault occurs and current flows to earth, the phase and neutral currents will no longer be balanced. A flux will be induced in the toroid and a current will flow in the secondary winding which will activate the trip mechanism and cut the incoming supply.

The trip mechanism operates when the residual current is between 50% and 100% of the rated tripping current, as required by the relevant British Standards.

I read that to mean any current flow in the neutral or earth will cause an imbalance, causing the RCD to trip.

http://www.powercords.co.uk/howrcd.htm

 

[C.R.A.Z.Y]

Yeah that's how an RCD works and I understand that, what's stumping me is the combined earth and neutral. In theory the neutral coil in the RCD and earth are at the same potential in a TN-C-S system ?!?!

If an insulation fault occurs and current flows to earth, the phase and neutral currents will no longer be balanced

But that's what I mean, if in a Tn-C-S system the neutral and earth are combined and effectively the same thing, how can the RCD differentiate a flow to earth from a flow to neutral ??

 

[Sliver]

Yeah that's how an RCD works and I understand that, what's stumping me is the combined earth and neutral. In theory the neutral coil in the RCD and earth are at the same potential in a TN-C-S system ?!?!

Yes they are at the same potential. But within the property, beyond the consumer unit, they are on separate circuits. So the RCD can differentiate between neutral and earth. Any imbalance causes a trip. At least that is my understanding.

 

[C.R.A.Z.Y]

It's the other way around from my understanding!
They are separated at the CCU, but the Neutral is tapped at the center of the transformer supply
(Separated is kinda misleading as they are ultimately still connected together, branching off would be a better term imho)

 

[Sliver]

Those diagrams relate to the situation outside the property. The RCD is monitoring what's going inside the property. Like I said, Earth and Neutral are on separate circuits within the property and the RCD simply monitors them for 'leakage'. Any leakage results in a trip, be it on the Neutral or the Earth.

 

[Bubo]

When I was about 8 I had the bright idea to investigate the inner workings of an old wall mounted electric heater which had never worked, I think basically I wanted to take it apart and see what was what. I realise now it was probably just the element that my parents never bothered changing, and were too tight to change anyway cos it would cost money. Anyhow, there was this big flash and I ended up thrown onto my bed about 4 yards away, still holding a partially melted screwdriver. Can still remember the flash and the landing on bed part, but nothing in between. Was pretty bloody lucky when I think about it now.

 

[tom_e]

When I was about 8 I had the bright idea to investigate the inner workings of an old wall mounted electric heater which had never worked, I think basically I wanted to take it apart and see what was what. I realise now it was probably just the element that my parents never bothered changing, and were too tight to change anyway cos it would cost money. Anyhow, there was this big flash and I ended up thrown onto my bed about 4 yards away, still holding a partially melted screwdriver. Can still remember the flash and the landing on bed part, but nothing in between. Was pretty bloody lucky when I think about it now.

I had similar playing with an old CRT monitor touched something I shouldn't have and shot off the thing, stopped playing with them after that

 

[C.R.A.Z.Y]

Those diagrams relate to the situation outside the property. The RCD is monitoring what's going inside the property. Like I said, Earth and Neutral are on separate circuits within the property and the RCD simply monitors them for 'leakage'. Any leakage results in a trip, be it on the Neutral or the Earth.

How can they be on separate electrical circuits when they ultimately connect with each other without any breaks in-between (they are at the same potential)
The earth block and neutral will obviously have continuity with each other as they are effectively the same conductor split into two?!?!?

Yes they split off too the MET and CCU's neutral supply but they are still electrically connected too one another?!?!

(Effectively the earth and neutral cables in a TN-C-S system are parallel cables to one another at obviously the same potential as they are connected to one another!)

 

[Sliver]

How can they be on separate electrical circuits when they ultimately connect with each other without any breaks in-between (they are at the same potential)
The earth block and neutral will obviously have continuity with each other as they are effectively the same conductor split into two?!?!?

Yes they split off too the MET and CCU's neutral supply but they are still electrically connected too one another?!?!

They are on separate circuits. That's why we have 3 pin plugs and sockets unlike the US and most other places which only have 2. Live, Neutral, Earth. 3 circuits.

In the TN-C-S system, each piece of electrical equipment has both a protective ground connection to its case, and a neutral connection. These are all brought back to some common point in the building system, and a common connection is then made from that point back to the source of supply and to the earth.

They only share a common connection outside of the property. Or more accurately, supply side of the consumer unit.

 

[C.R.A.Z.Y]

They only share a common connection outside of the property. Or more accurately, supply side of the consumer unit.

But it doesn't matter where the common connection occurs as provided their are no breaks in either of the conductors after this point they will remain connected/at the same potential

My head hurts

In theory if the Neutral from the supply is lost then it will create a dangerous situation as all the earth bonding/metal casings will now be supply live (240 volts) with regard to earth due to the floating neutral ?!?!

 

[bitslice]

Oh and one nice thing about electric shocks... you may feel fine for ages after, then drop dead.

Something to do with heart rhythm from what I can remember being messed up

I hadn't thought of that,
surely this would imply that if anyone has received an electric shock at work, that they should then also have an ECG to check for arrhythmia? (Elf n safety)

 

[Sliver]

But it doesn't matter where the common connection occurs as provided their are no breaks in either of the conductors after this point they will remain connected/at the same potential

My head hurts

Yes they remain at the same potential unless a fault develops within the property. Then that fault current will try and make it's way back to Earth, but to do so it must pass the RCD which detects it and then trips the circuit.

In theory if the Neutral from the supply is lost then it will create a dangerous situation as all the earth bonding/metal casings will now be supply live (240 volts) with regard to earth due to the floating neutral ?!?!

No. The RCD would detect the differential between Neutral and Earth and trip the circuit. Neutral and Earth must remain at the same potential to avoid triggering the RCD.

 

[new boy]

I've had 2 electric shocks from my light switch in the bathroom. I cant believe someone thought it would be a good idea to put it above the sink. I'm aware this is against pretty much every building reg ever made lol. Should be a pull switch etc.

That was about electric fence bad. nothing tripped out. Would be happy to recommend it, the lols outweighed the minor pain.

Not sure I'd fancy the 13A sockets though. I'm glad there aint one of them above the sink.

 

[Terminal_Boy]

It's not something you forget is it. I've been ultra cautious ever since my zap. My dad also a sparky, now retired, says if you've never had a shock you've never been doing the job. Happens to the best of us.

No chance of forgetting that. Over 25 years ago now and I've treated everything above low Voltage DC with the respect/caution it deserves ever since.

 

[Vern1961]

Had a few shocks over the years.

First at an Uncle's house with really ancient wire, must have been an earth leak as with one hand in the bathroom basin, touching the cold tap with the other gave me a shock.

Next was, "wonder what happens if you put a 9v PP3 across the +- plug terminals on a model railway transformer." Somewhat more than 9v came back... Not sure how much but enough to penetrate the battery case and give me a sore arm for a few days.

Back EMF off the vacuum is a favourite for a nasty tingle if you touch the plug while the motor is running down.

And last year I had the carpal tunnel shock tests done (posted anecdotally not as a medical query/advice), that got quite uncomfortable at the latter stages.

 

[CorrectHorseBattery]

I'm rolling on the floor OP. Good work with this thread.

 

[mattyg]

I had similar playing with an old CRT monitor touched something I shouldn't have and shot off the thing, stopped playing with them after that

Ahaha From the LOPTX it goes to the cap on the top of the CRT.. Thats where you get the 20-25000volts from. Not a lot of power but can put a hole in ya finger.