Concerned.

[tom1]

quick question how come testing ir at 1000v?

 

[steptoe]

But one disadvantage of TT is that if for some reason an RCD fails you will have no earth protection. With an earth resistance of 100 ohms an earth fault current would be just over 2 Amps, this would not trip any MCB and any metal would be at 230 volts until someone touched it.

RCD's are meant as a backup to MCB's or fuses and not as a primary and only source of earth fault protection.

That's the way I was taught anyway.

so how would PME/TNCS be any different?

an RCD in any installation is primarily there for earth fault protection if someone should touch it.

do you understand the difference between direct and indirect contact?

(if this name has changed can someone please correct me)

MCBs are designed for overcurrent protection,

RCDs are designed for earth fault protection

the earthing system is irrelevant in both cases

 

[Andy™]

With an earth resistance of 100 ohms an earth fault current would be just over 2 Amps, this would not trip any MCB and any metal would be at 230 volts until someone touched it.

no. it wont be at 230v. it will be much lower (after all, touch voltage should be limited to 50v by design)

& steps - its now basic & fault. but direct & indirect were better

 

[la poste]

So if I have a circuit say a lighting circuit with metal switches, The RCD fails and so I am relying on MCB protection.

I get an earth fault on the light and the switch becomes live, The live current will flow back to the board and down the earth rod at roughly 2 Amps.

This will not trip a 6A MCB. So the light switch will be at 230V until someone touches it.

Where am I wrong with this argument?

 

[steptoe]

So if I have a circuit say a lighting circuit with metal switches, The RCD fails and so I am relying on MCB protection.I get an earth fault on the light and the switch becomes live, The live current will flow back to the board and down the earth rod at roughly 2 Amps.

This will not trip a 6A MCB. So the light switch will be at 230V until someone touches it.

Where am I wrong with this argument?

and the difference with an RCD failing on PME is.?

on a correctly designed system an earth fault would normally overcurrent the MCB.

I know I have seen it happen where it does not, and I have got some serious shocks L-N from RCD protected systems as well.

nothing is foolproof, we just have to design and install as best we can,

if you are so worried about RCD failure install half a dozen of them.

 

[steptoe]

Well you need to remember that we need to create an equipotential zone, so there should be little risk of hand to foot shock. the risk would be between hand to hand where we may be in contact with an exposed conductive part and simultaneous in contact with an extraneous conductive part which is not bonded at therefore at different potentials.

wish I was a plumber and able to put into words,

or at least be able to think the pure basics of what I would have thought was known to someone who was even talking this in depth about earthing systems.

 

[la poste]

Well you need to remember that we need to create an equipotential zone, so there should be little risk of hand to foot shock. the risk would be between hand to hand where we may be in contact with an exposed conductive part and simultaneous in contact with an extraneous conductive part which is not bonded at therefore at different potentials.

Excellent cheers. Equipotential zones, I guess with a TT they are extra important.

and the difference with an RCD failing on PME is.?

If an RCD fails on PME and there is an earth fault the earth fault resistance is so small it will allow a very high fault current to flow tripping the MCB's in the correct time and cutting the circuit dead.

With a TT the earth fault path resistance is so high it would never trip any MCB. MCB's would only trip on a phase to neutral fault not a phase to earth fault and the bonding at the board would help spread this potentially dangerous and undetected fault voltage around the house.

Great discussion by the way.

 

[Chr!s]

If an RCD fails on PME and there is an earth fault the earth fault resistance is so small it will allow a very high fault current to flow tripping the MCB's in the correct time and cutting the circuit dead.

With a TT the earth fault path resistance is so high it would never trip any MCB. MCB's would only trip on a a phase to neutral fault not a phase to earth fault.

This is true

But a well designed tt system is safe.

 

[steptoe]

If an RCD fails on PME and there is an earth fault the earth fault resistance is so small it will allow a very high fault current to flow tripping the MCB's in the correct time and cutting the circuit dead.With a TT the earth fault path resistance is so high it would never trip any MCB. MCB's would only trip on a a phase to neutral fault not a phase to earth fault.

This is true

But a well designed tt system is safe.

your arguement only stands good until you lose a neutral, which is a bigger probability than having a failed RCD,

then no matter what happens on a PME everything that is earthed will go to 230v potential and will not be able to blow either RCD or MCB until someone touches it.

 

[batty]

your arguement only stands good until you lose a neutral, which is a bigger probability than having a failed RCD,then no matter what happens on a PME everything that is earthed will go to 230v potential and will not be able to blow either RCD or MCB until someone touches it.

Steps I am trying to understand this on a modern house with plastic services. If you loose the incoming neutral surely nothing will work because the circuit is no longer a circuit. The neutral and cpc's are seperate on consumer side so how can the earth rise to 240 volts as it seperate to the neutral?

 

[Lurch]

I think PME is better than an earth spike, does anyone disagree with anything I have said?

My heart rate doubled when I read that, then I looked at the page count. I almost stopped reading that mid sentence and ran away.

If you loose the incoming neutral surely nothing will work because the circuit is no longer a circuit. The neutral and cpc's are seperate on consumer side so how can the earth rise to 240 volts as it seperate to the neutral?

If the neutral is lost on the network then you also lose the earth so the metalwork potential rises to 230V because of the N-E link in the head.

If you simply lost the neutral between the head and CU then what you say is correct.

 

[steptoe]

I am starting to think this is going the way of many previous threads and would appreciate someone providing the appropriate link.

if the poster is still suggesting that PME/TNCS is better than TT then perhaps it is time for me to stop posting in this thread.

I think my opinion has been made clear enough many times before.

 

[steptoe]

My heart rate doubled when I read that, then I looked at the page count. I almost stopped reading that mid sentence and ran away.If the neutral is lost on the network then you also lose the earth so the metalwork potential rises to 230V because of the N-E link in the head.

If you simply lost the neutral between the head and CU then what you say is correct.

a poorly designed installation that no matter the earthing arrangements would make no different.

 

[SPECIAL LOCATION]

Yes, Steptoe.I feel another long thread coming on.........more Guinness required.

Dont you dare drink all the ruddy GuinnessGuinnessGuinnessGuinness

or I am telling on you...

ADMIN

Pclec's nickin all the beer!!!! :_| :_| :_| :_|

 

[la poste]

A link would be great, this is new to me cheers.

 

[Apache]

A link would be great, this is new to me cheers.

My thread http://www.talk.electricianforum.co.uk/showthread.php?t=1559

The big one that came before http://www.talk.electricianforum.co.uk/showthread.php?t=483

 

[tom1]

My thread http://www.talk.electricianforum.co.uk/showthread.php?t=1559The big one that came before http://www.talk.electricianforum.co.uk/showthread.php?t=483

well that will be a few hours of la time

 

[SPECIAL LOCATION]

well that will be a few hours of la time

Did I tell you I have memorised BOTH of those threads off by heart..

I can quote em verbatim word for word!!!

Go on ask me..

what the 475th word?????????????????

O) :Blushing

Has admin been round yet with the medication trolley yet?



if yes.....

think he missed me again?

:_| :_| :_|

 

[tom1]

Did I tell you I have memorised BOTH of those threads off by heart..I can quote em verbatim word for word!!!

Go on ask me..

what the 475th word?????????????????

O) :Blushing

Has admin been round yet with the medication trolley yet?



if yes.....

think he missed me again?

:_| :_| :_|

deke and brian allways scof all the meds befor anyone gets em

 

[steptoe]

well that will be a few hours of la time

:slap :yawn