Exterior socket on PME system

[happysteve]

It's RCD protected...

It's solving the problem of a lost neutral on the DNO side.

Even then an Ra of 3.35 isn't a huge effort.

I'm confused. Surely an RCD won't help for additional (shock) protection when there's a missing PEN (on the DNO side)? Current flows out through the L, through the RCD, through the load, back through the N, same current back through the RCD (so, balanced), to the PME block (where earthing conductor is connected to PEN), to MET and through CPC to exposed conductive parts, through body of person, to earth.

I agree that having an earth electrode with good Ra is better than not having one (for PME) to reduce the touch voltage in the event of loss of PEN on the supply side. To remove the shock risk requires a very low Ra, and something like 3.35Ω, depending on the local ground conditions, may be... challenging.

 

[Bobster]

Draw it out Steve, the N>E link is before the RCD, so current flowing down the PE will not go through the RCD causing an imbalance.

 

[Midwest]

Are you both not saying the same thing?

 

[hightower]

On quickly reading and passing through, I've got to agree with happy steve.

Scenario: Class 1 appliance, no faults present anywhere on installation except broken neutral on PEN conductor. Current is balanced through RCD, earthed conductors rise to a dangerous voltage, link between Class 1 appliance and earth made via a person, RCD is going to say "hey, this looks alright to me"

What am I missing?

 

[Midwest]

Don't know, my understanding, limited as it is, is broken PEN conductor, is your worst case scenario (don't have nightmares folks) RCD or no RCD.

 

[South Spark]

Realistically the place has metal pipework disappearing underground that will have a lower resistance than any bit of copper I could bang into the ground.

 

[hightower]

Realistically the place has metal pipework disappearing underground that will have a lower resistance than any bit of copper I could bang into the ground.

Which you can't utilise in your design of this addition.

 

[soulman]

Hi, An Ra of 3.35 is not easily achievable. I have had 6 rods in the ground & managed 20 ohms

 

[Midwest]

It's remarkable how some threads go off on a

 

[soulman]

Apologies, I was just surprised at that comment.

 

[Midwest]

Apologies, I was just surprised at that comment.

Which one?

 

[soulman]

Achieving an Ra of 3.35

 

[happysteve]

Draw it out Steve, the N>E link is before the RCD, so current flowing down the PE will not go through the RCD causing an imbalance.

Okey dokey The squiggles inside the circle are meant to represent an RCD. In the event of a broken PEN (and no other fault), the same current passes through both poles of the RCD and there is no imbalance. The current then flows through the CPC, through the person, and to earth.


The picture below just shows that the formula A722.3 is correct.


So I still maintain that an RCD will not help in this situation.

 

[Midwest]

I'm cooking 'tea' at the mo, so not following the thread, but this vid (Pete's copyright) is often used;

 

[South Spark]

Ok long day so bear with me. Informative video but it seems to me that it only applies if there is no other path to earth than the affected person.

If the PEN is broken then would the current not flow back through the bonding to true earth especially if I introduce earth rods into the system?

Could I introduce an earth rod connected to the earth terminal in the socket?

 

[Midwest]

It's a matter of timing

 

[Des 56]

Utilise the Pme for the socket,add a rod

Don't employ any Horses,Donkeys or Cows to operate the sound system and every ones happy

 

[happysteve]

Ok long day so bear with me. Informative video but it seems to me that it only applies if there is no other path to earth than the affected person.

If the PEN is broken then would the current not flow back through the bonding to true earth especially if I introduce earth rods into the system?

Could I introduce an earth rod connected to the earth terminal in the socket?

Sorry South Spark for meandering off a bit...

Re your first question... yes it would. There has been some to-ing and fro-ing about the numbers, but essentially yes, having an earth rod would make the situation safer.

Re your second question... If you mean, "connect the earth rod to the socket and still retain the CPC from the TN-C-S" I would advise against this, unless you have 10mm earthing conductor all the way back to the MET (for the same reason that bonding needs to be 10mm). If you mean, "Run the CPC to the socket but don't connect it to the earth pin (or anything exposed) and connect the earth pin to a local rod," then this sounds like "TT-ing your shed" without a sub-board. The protective device (and additional protection) would be on one earthing system, and the socket-outlet on another. I don't know if this is relevant or not, and I've reached the limit of my knowledge about whether this would be permitted or advisable.

I suppose the way of complying to keep everyone (incluing your CPS advisors) happy would be to treat this 32A socket as a separate board and TT it. So install an RCD garage subboard with the socket nearby, put this sub-board on the earth rod, and follow the usual procedure for making sure exposed conductive parts from the two different earthing systems are not simultaneously accessible (411.3.1.1).

 

[South Spark]

Utilise the Pme for the socket,add a rod

Don't employ any Horses,Donkeys or Cows to operate the sound system and every ones happy

Where are you talking about adding a rod? To the socket (disconnect PME earth) or to the MET?

 

[davesparks]

If it is to be used for outdoor events then you'd do well to read up on the requirements of BS7909 'code of practice for temporary electrical systems for entertainment and related purposes'
This is becoming more commonly recognised in the industry, and councils are increasingly requiring compliance as part of event licensing.

As it stands you may find you have a grumpy customer on the phone when the event electrician comes along and refuses to use the socket as it is non-compliant.