Novel Slant on a PME

[didthathurt]

Saw a novel slant on the theory of a PME system today. Class 1 light fitting on a non-cpc lighting circuit with the fitting earth lead connected to the neutral terminal of the supply.

 

[securespark]

Thou shalt not connect neutral to earth within the property...

 

[plugwash]

i belive that method of earthing used to be common in east germany.

 

[RF Lighting]

Surely that is TN-C earthing?

 

[Spark123]

It is also slapped wrists with the ESQCR, "A consumer shall not combine the neutral and protective functions in a single conductor in his consumer's installation."

 

[PompeySparks]

Agree Spark123 - also phohibited by BS7671 (546-02-08 )

 

[ricicle]

I once saw a fluorescent fitting on a system with no earth and it had been fitted with a diode between the metal body and neutral

 

[Adam_151]

I'm told by a rather old lecturer that TNC systems were used on some department store buildings in the 1960's using single core MI, I guess the reduced voltage drop in the CNE sheath due to its much larger CSA, combined with the un-likelyness of it being broken made it somewhat acceptable. Apparently the 17th is going to completely get rid of what small references remain to TNC systems

And yeah, I've also heard that re-creating earth from neutral was common in eastern europe. Contrast that to the US, where 'cheater plugs' are a common solution to a three prong plug and two hole socket (and as long as you have retro-fitted GFCI protection its considered ok in old installations), granted they use 10ma for general purpose where as we use 30ma, and their voltage from a live conductor to ground is half what ours is* I'm not sure exactly how safe this works out in practice, but I can't imagine it 'gets' many people or they'd do something about it!

*At least domestically where they have one 220v phase centre grounded, but they also have more interesting arrangements such as three of them in delta, and leaving the ground on the centre of the first one, resulting in two conductors 110v above ground, and the third 208v above ground!

 

[RF Lighting]

I seem to remember hearing somewhere that TN-C was often used on lighting circuits in mines.

 

[Adam_151]

I seem to remember hearing somewhere that TN-C was often used on lighting circuits in mines.

I thought it was IT that was common in mines?

 

[ricicle]

I thought it was IT that was common in mines?

No, they don,t use PCs the coal dust gets in the keyboards

 

[plugwash]

I'm told by a rather old lecturer that TNC systems were used on some department store buildings in the 1960's using single core MI, I guess the reduced voltage drop in the CNE sheath due to its much larger CSA, combined with the un-likelyness of it being broken made it somewhat acceptable.

it certainly doesn't sound any worse than using copper or aluminium concentric with a live down the middle and CNE arround the outside which is what the recs do.

Note that there is an exception in the ESQCR, if you are operating an electric guided transportation system (a railway or tramway) you are allowed to use a combined neutral and earth.

If a system using CNE cores is well installed and maintained using appropriate cable types then it should not pose a significant safety risk. Trouble is most real installations (even commercial ones) see some degree of neglect and bodges by cowboys and/or diyers.

how often do you sparkies work on circuits that are only single pole isolated, my guess is quite often. Do you feel at risk while doing so? have you ever had a shock while doing so? do you trust the neutral to be safe to touch?

*At least domestically where they have one 220v phase centre grounded, but they also have more interesting arrangements such as three of them in delta, and leaving the ground on the centre of the first one, resulting in two conductors 110v above ground, and the third 208v above ground!

quite a sensible setup actually, lets you mix 120V single phase, 240V single phase and 240V three phase loads on the same supply system. Main downside is you cant balance it properly unless your total 240V load is more than double your total 120V load.

 

[RF Lighting]

how often do you sparkies work on circuits that are only single pole isolated, my guess is quite often. Do you feel at risk while doing so? have you ever had a shock while doing so? do you trust the neutral to be safe to touch?

Good point.

Normally I am quite happy working on SP isolated circuits, but am in the habit of testing the voltage on the neutral conductor every time a joint / cable is cut or broken.

I have been got by neutral feedback, hence the habit. It was the good old live from upstairs lights neutral from downstairs lights scenario.

It only takes 2 seconds to check. It could be the difference between life and death. Something we have great difficulty getting through to one of our apprentices.

 

[plugwash]

I have been got by neutral feedback, hence the habit. It was the good old live from upstairs lights neutral from downstairs lights scenario.

hmm? i thought it was usually live from downstairs and neutral from upstairs

 

[RF Lighting]

You know what I meant.