3 DNO in school buildings

[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