3 DNO in school buildings

[cliffed]

EICR finding 3 DNO supplies in main building.
The front end of building 3p 100 amp TNS, middle Sp 100 amp TNS, other part Sp 100 amp TNCS.
Water & Gas supplies are common to this building
This been in situ for over 50 years, any comments

 

[James]

At each main switch (for each supply) I would like to see a warning sign that says building is fed from multiple supply’s
A simple building plan showing where they are would be good too.

It is one thing the fire brigade wanted in a building I managed many years ago that had 2 supplies

 

[Lister1987]

Imagine school has its own substation so ultimately it's all the same supply, just different phases (in thr case of single phase) and same origin for 3 phase. Multiple heads but same source. Not uncommon when you think about that logistics of it.

One of the warehouses I was at earlier in thr month has 3 onsite substations, which multiple heads/intakes and not a single multi phase stickers in sight owing to suitable design keeping any case of touching multiple phases to a minimum, if at all.

 

[westward10]

There is no way of knowing without further information if they are from the same source. I would suggest the main earth terminals need bonding together to a common main earth terminal along with the main protective bonding.

 

[Lister1987]

There is no way of knowing without further information if they are from the same source. I would suggest the main earth terminals need bonding together to a common main earth terminal along with the main protective bonding.

Was going to make the foolish suggestion that there should be 'as fitted' drawings available either on site or from the LEA but thought better of it.

 

[cliffed]

Imagine school has its own substation so ultimately it's all the same supply, just different phases (in thr case of single phase) and same origin for 3 phase. Multiple heads but same source. Not uncommon when you think about that logistics of it.

One of the warehouses I was at earlier in thr month has 3 onsite substations, which multiple heads/intakes and not a single multi phase stickers in sight owing to suitable design keeping any case of touching multiple phases to a minimum, if at all.

No it’s definitely 3 DNO supplies the old type cable with sheath earth & the other a new TNCS

 

[timhoward]

If it's been like this for 50 years I'd imagine the main earth conductor sizes and main bonding conductor sizes are smaller than they would be today?

I also think that bonding the METs together seems a good solution.
At an absolute minimum I think the main bonding should be sized as though it was TNCS at the two TNS supplies, but if the METs were bonded then a lower resistance path than the water/gas pipes exists for any diverted neutral currents arising from a TNCS issue.

 

[cliffed]

There is no way of knowing without further information if they are from the same source. I would suggest the main earth terminals need bonding together to a common main earth terminal along with the main protective bonding.

Good shout i think, maybe worth noting on report, I doubt any classification codes can be give maybe FI

 

[westward10]

FI is a Code.

 

[snowhead]

Maybe contact the DNO and ask if they can confirm the source for each individual supply.
It could be that the single phase supplies are tapped off the 3ph supply or all come from a cabinet / pillar onsite.
Are both single phase the same or different phases?

 

[cliffed]

Maybe contact the DNO and ask if they can confirm the source for each individual supply.
It could be that the single phase supplies are tapped off the 3ph supply or all come from a cabinet / pillar onsite.
Are both single phase the same or different phases?

Not a clue on the phasing of the SP no coloured discs

 

[Simon47]

My first thought in this sort of situation is regarding separation of systems.
Given Lister1987's comment regarding "as installed" drawings, there's probably a real risk of multiple supplies being present in some part of the building. Apart from there being different earthing systems, there's then the risk of someone turning off one supply and being "surprised" to find something still live.
And then there's the situation with systems which intentionally cross dividing lines - e.g. fire alarms, school bell system, ...

 

[UNG]

No it’s definitely 3 DNO supplies the old type cable with sheath earth & the other a new TNCS

So all three supplies are separately metered

 

[cliffed]

So all three supplies are separately metered

Yes all have separate meters

 

[UNG]

If it's been like this for 50 years I'd imagine the main earth conductor sizes and main bonding conductor sizes are smaller than they would be today?

I also think that bonding the METs together seems a good solution.
At an absolute minimum I think the main bonding should be sized as though it was TNCS at the two TNS supplies, but if the METs were bonded then a lower resistance path than the water/gas pipes exists for any diverted neutral currents arising from a TNCS issue.

Given the different supply types I would be asking the DNO for their opinion on bonding the MET's together
It could get a bit messy with varying supply loop impedances and mixed earthing methods especially if the supplies are coming from different substations, there is also the consequences if the school is supplied from different subs of one substation supply losing it's earth and becomes earthed from another substation via the bonded MET's at the school