Mixing TT and TNCS supply systems

[LastManOnline]

Was informed of this incident last week. A row of period houses in an older part of the city. All on TT earthing. One gets rewired. TNCS is obligatory for all new installations. DNO suffers a neutral break close to the trafo. All 23 houses now divert their neutral current back through the neutralised houses main (and supplementary) bonding. As it was an older part of town the metallic services were all intact and provided a low resistance path back to the trafo. It was estimated this home had up to 700 amps travelling through it at one point. Though no fire occurred there was massive smoke damage resulting in huge damage.

Two lessons for me.
.1) Check the neighbours supply system when doing rewires.
2) Consider electrically isolating any main metallic services still in use

 

[Mikegh]

It's an 80 amp DP isolator.

If there's a main overcurrent protective device upstream in the cabinet

 

[pc1966]

On coming across a TT supply, a REC (registered electrical contractor) is obliged to issue a "Notice of Hazard" to the homeowner, alerting them of the requirement to upgrade to TNCS. No exceptions are allowed.

Has the nature of the hazard been described?

Is it this very issue of TT'd homes returning open PEN current via another's neutralising link?

 

[LastManOnline]

Has the nature of the hazard been described?

The description of the hazard would simply be termed as the home being "un-neutralised".

Is it this very issue of TT'd homes returning open PEN current via another's neutralising link?

Yes. That's one issue. But main issue is the gradual disappearance of the metallic network that was deemed essential for a sustainable TT system.

 

[marcuswareham]

These systems only disconnect the supply if the L - N voltage is outside of range of 207v - 253v it is still very possible to have a PEN fault with a dangerous voltage present on all exposed metal work and these devices would be non the wiser, therefore they do not protection against PEN faults in everycase

Other products have a device which senses any current flowing on the CPC, these require a person or animal to receive a shock for a very short duration in order for them to disconnect in a PEN fault scenario

So as yet we do not really have a satisfactory single phase PEN fault detection/protection device

 

[Mikegh]

These systems only disconnect the supply if the L - N voltage is outside of range of 207v - 253v it is still very possible to have a PEN fault with a dangerous voltage present on all exposed metal work and these devices would be non the wiser, therefore they do not protection against PEN faults in everycase

Other products have a device which senses any current flowing on the CPC, these require a person or animal to receive a shock for a very short duration in order for them to disconnect in a PEN fault scenario

So as yet we do not really have a satisfactory single phase PEN fault detection/protection device

A protective device that requires a person to receive a shock

Hmm


Not sure I like the sound of that

 

[pc1966]

The description of the hazard would simply be termed as the home being "un-neutralised".

Which actually says nothing about a risk!

Yes. That's one issue. But main issue is the gradual disappearance of the metallic network that was deemed essential for a sustainable TT system.

I thought the whole point of the earth rod in a TT system is it is sufficient for disconnection, so the bonding to service pipes is not essential?

Not that we don't have stupid rules with little or no justification here of course...

 

[LastManOnline]

Which actually says nothing about a risk!

Correct

I thought the whole point of the earth rod in a TT system is it is sufficient for disconnection, so the bonding to service pipes is not essential?

Correct again. All of these decisions (regarding change of the supply system) are taken without any consultation of the "rank and file". They are simply handed on down.

 

[LastManOnline]

I thought the ROI now mandated double-pole MCB as the main incomer switch,

You were right.A DP 63 mcb will be used in existing installs where the tails are less than 3 meters. The 80 amp DP isolator will be used in newer installations which already have overload protection at the meter. Just got the update. You were ahead of me...... Again!!!!!

 

[Mikegh]

You were right.A DP 63 mcb will be used in existing installs where the tails are less than 3 meters. The 80 amp DP isolator will be used in newer installations which already have overload protection at the meter. Just got the update. You were ahead of me...... Again!!!!!

Seems to be a lot harmonization now between the UK and Ireland

Isn't the voltage drop changing to 3/5 for lights and power

The DP incommer I always thought was a no brainer

 

[LastManOnline]

The DP incommer I always thought was a no brainer

Completely agree. It would have my life so much easier on countless occasions had it been in place earlier

 

[Mikegh]

Completely agree. It would have my life so much easier on countless occasions had it been in place earlier

Only thing I don't like about the UK rules is the dual rcd board, sharing lighting and power on an rcd

Imo lighting should be MCB or rcbo