College assigment help - shock from non energised circuit?

[sparkybird]

Hi everyone and a happy easter!

I don't normally like to post college questions here, coz the whole point is to work them out yourself, but I wondered if you could have a look at my suggested anwer to this one?

Single phase new install. One of the ring final circuits is energised to enable IT equipment testing to be carried out. When connecting a socket to a different circuit (isolated from supply) an electric shock is received when touching phase and cpc conductors at the same time.
Explain why this occured and how it could have been avoided.

From the rest of the assignment, I think we assume that the circuits have all been tested out OK.

My thoughts are that IT equipment can have high cpc currents, so that there could have been current in the cpc and since all cpc's are connected at the fuse board, this could have produced a potential difference between the other socket circuit cpc and phase (at 0v).
To avoid this - disconnect the cpcs of the circuit being worked on from the fuse board but not forgetting to reconnect before energising.

Am I on the right track, do you think?

Hope you can help
SB

 

[sparkybird]

29 views and no opinions?

Or maybe you're all too busy scoffing your easter eggs!!

SB

 

[Jim2287]

if the CPCs become live, that makes the metal parts live too

i dont think disconnecting CPCs while you work on it is the resolution, since you shouldnt be getting a shock off CPC's

i think you should improve EFLI at supply earth so the potential doesnt build up, preventing electric shock
touch voltage shouldnt be above 50volts (25volts in agricultural installations)

 

[dingbat]

To avoid this - disconnect the cpcs of the circuit being worked on from the fuse board but not forgetting to reconnect before energising.

Better to connect the phase of the isolated circuit to the earth bar. At least this way there is not only no potential differencebetween them, but you can't energise the circuit leaving the CPC disconnected.

 

[sparkybird]

Thanks for replies folks!

Jim - I'm not sure if I catch your drift? Before energising the IT circuit the EFLI would have been tested out and been OK, so not sure how improving it would help? Or am I missing something?

Dingbat - good point, but could you then get a pd between neutral and cpc/phase?? Do you think my high cpc current idea is why there is a problem?

Cheers

SB

 

[Adam_151]

Agree with ding that the best way is to connect the disconnected phase to the other bar.

Another possible course for the problem is if the diconnected circuit is run in the same trunking/conduit as an energised one, you'll get capacative coupling between them, would have to be together for some considerable difference for a shock to be felt though, this type of situation usually manifests itself as a reading on a high impedance digital volt meter, or lighting of the imfamous neon screwdriver

Doing a few quick calculations on the back of a box from a chocolate egg... if you had a piece of 1mm T&E and energised the phase and left the earth disconnected, you'd need over a km of cable to get a shock of the disconnected earth (I'm using a value of 10ma for 'shock' here) and even then I'm assuming near zero body resistance, which wouldn't happen in reality. Obviously different cable configurations will have different values of capacatance (I'm using 100pf per metre in my calculations, as a value given to me by someone else)*

Or it is possible that the disconnected cable had been meggered and the charge stored in the capatance of the disconnected circuit had not been discharged properly

*Impedance is equal to 1 / (2pi x freq x cap) a quick rearrangement of this is all that is needed to give the value of capacitor needed to give a certain impedance at a known frequency

 

[RF Lighting]

I think you are could be along the right lines regarding the the IT equipment producing the high voltage / current on the CPC. I would however have expected that if the main earth is connected to the installation then the voltage should flow to earth. There would be no P.D. between the phase and CPC conductors as the phase conductor is not connected to anything so the voltage would have no where to go. The only other possible cause I can think of is if the cables for the new circuit run along side the cables for the IT circuit (possibly a run of singles in trunking and conduit) then there could be a voltage being induced into your phase conductor, thus creating the voltage to earth through your connected CPC. But then again I could be barking up completly the wrong tree.

Rob

 

[sparkybird]

Hmmm - more food for thought!

The install is wired in pvc insulated singles in pvc conduit and it's a betting shop (comedian, my college teacher). There are 10 fluorescent lights, if this is of any interest.

Thinking again about the high cpc currents, as this is a betting shop there won't be masses of IT equipment, so would you indeed get this problem? Also see your point, RF lighting, that if earthed there shouldn't be a pd anyway! Maybe the install is not as well earthed as we think? However, I reckon EFLI test would have shown this. To get induced voltage in the phase cable of the 'shock' circuit, would you need to have the IT circuit neutral running elsewhere ie not in the same conduit?


Re the meggering, Adam, is this common? I've not come across it.

Eaten far too much choc and need to have a lie down now

Thanks again, all

SB

 

[Jim2287]

with the EFLI, what i was getting at was what RF lighting said...
the earth leakage current should just go down to earth, it shouldnt build up a voltage high enough to give an electric shock. the EFLI might have been acceptable for disconnection of the breaker in a fault, but not sufficient to allow the amount of the earth leakage of the computer equipment to flow freely to earth



i thought about the induced voltage RF lighting, but would it be able to build a voltage high enough to give electric shock ?


Stored voltage of the Megger test is quite common, I always get it done by that one ... if that was the problem all you would have to do is touch the conductors together to let the voltage dissipate

 

[RF Lighting]

The only time I have experienced a capacitive charge after meggering was with a piece of pyro, and I think it is unlikely that PVC singles could hold a charge for much of a period of time.

Can we have an easy one next time please sparkybird

Rob

 

[sparkybird]

Thanks for all your replies!

Since we didn't have a major consenus of opinion, I'll check back with my college teacher after the easter break to see if we're on the right lines.

And yes, I'd like an easier one next time! (the other 24 questions on the assignment were OK)

Enjoy the rest of the weekend

SB

 

[dingbat]

If it's any help, SB, City & Guilds questions are often poorly written, so they are often nowhere near as difficult as they seem to be. In this case I can't think of anything that hasn't been covered in other answers.

When I did my 2360 I often handed in ten-page written and illustrated reports to fully answer ambiguous questions from all possible angles, only to find out that the stock answer to the question the examiner thought he was setting was something like "red" or "4", or "with a screwdriver". My lecturer loved me!

 

[Dellsmp]

Is the IT equipment still connected to the cct? IT stuff has large caps( to prevent surges) between phase and cpc, so they will hold a charge long after the circuit is isolated, just a possibility. I used to regularly find very high latent charge on smoothing caps on power supplies when I worked for maint in a factory. Used to short out the cap terminals with screwdriver and watch the sparks ate screwdrivers tho