2 supplies in 1 property

[Judd]

I carried some work out today in a property in a block of flats, just installing a fan in a bathroom. whilst carrying out some tests I noticed an MCB in the DB marked coil. After some investigation and discussion with the owner it sounds like (I can't be certain)
This MCB feeds a contactor in the communal service cupboard which in turn feeds her storage heaters, so effectively her storage heaters aren't fed from her own supply. I couldn't gain access to the service cupboard as it was locked and i couldn't obtain a key at the time.
My question is in the event of a fault could this create a dangerous pd across the casings of the storage heaters?
Cut out was located inside the flat
tn-cs
Thanks

 

[davesparks]

What do you mean by a dangerous pd across the casings of the heaters?
The casing will be a metal sheet so the pd between any two points on it would be as near to zero as dammit.

 

[RWJ]

Not really getting this, tbh it might be because it's very early and I can't sleep!

Is your question relating to...
A. They potentially could be fed from different phases,
B. Due to the earth coming from different supply cables.
C. Something else?

A. = It would just be like a TP install IF there were off differing phases.

B = As Dave's answer the difference in resistance would be negligible for the PD to ever get that high.
As a quick indication as you didn't have access to the other supply. You could have taken a ELI using a nearby socket then taken it again (same socket) but using the heater as earth. This would have given you an idea of the impedance difference thus you could then work out the potential difference.

 

[johnduffell]

My issue would be lack of access to the main switch for the installation. If there are two cutouts and each has a separate connection between cne and the main earth(s) then there would also be issuei with the integrity of the equipotential zone. Perhaps that is what the op is referring to.
If there is a current flowing in the cne between the two links then it would be tricky to bond. But a pme install has currents flowing through bonding conductors anyway so bonding them all together would be possible even if not compliant, assuming you could get access to the cutouts and main earth terminals for each.