Shower trips another flat`s supply?

[festa]

Got back to the office today to hear of a strange problem some of our guy`s were called to...any idea`s?


A house converted to two flat`s,one up,one down.
Each flat has been given it`s own supply TNC-S and each flat has a consumer unit,metal clad with 100A 30mA main isolator.
When the tennent`s upstairs uses their shower and it`s been running 5 mins or so the RCD trips AND takes the lower flat`s RCD out too.
All circuit`s were tested (in both flat`s) and result`s came up satisfactory.
We put it down to a supply problem,damaged neutral maybe but the local DNO came out today and said no problems could be found.

ANY IDEAS ???

 

[JohnD]

no, but I'm thinking an old house converted into flats will probably have bits from one flat connected to the other.

E.g. water and drain pipes via supplementary bonding, possibly old conduits under the floor and old gas pipes. there might even be a borrowed neutral from one house going to another, e.g. on the stairs lights

 

[plugwash]

It's a long shot but one possibility would be a high resistance fault between the two flats live's or neutrals. The heavy load of a shower could drive extra current down such a fault tripping both RCDs.

I would suggest turning off the main switches for both flats, then run an extention lead from one flat to the other and start meggering to see if you can detect any high resistance cross connections.

 

[jj4091]

Maybe a stupid question but do they have water leaking from the shower into upstairs & downstairs electrics? Another thought would be that if it happens every time the shower is used, could you isolate it to a particular cct. in downstairs flat by turning off all the mcb.s & switching back on 1 at a time until the rcd trips. I know that both rcds tripping tend to rule this out, but you may have more than 1 short.