The method i find best is not to energise, but test for your r1 + r2 and calculate your Zs. As you have the Ze and r1 + r2 reading you can easily calculate Zs
I would concur with the above method as the easiest.
As by calculation or measurement are both perfectly acceptable.
for either option you still have to open up the connection at storage heater
e.g. DP switch or FCU whatever is fitted. to put your meter leads on.
Read you Ze at main property CU...
this will have negligible difference to the true Ze at storage heater CU..
(assuming both are mounted within a couple of feet of each other?)
just strap your phase & CPC at the storage heater CU.
then read R1+R2 at each heater.
add them up jobs done!
Also considering that most final circuits to the heaters are generally reasonably short runs...
it is very rare that you are anywhere near max Zs values so a slight discrepancy of say 0.01ohm don't make no difference!
But saying that....
most of the work I do on night storage heaters is ....
Taking them out!
:^O:^O