Confused with Zs calculation

[st3v3]

Hi guys

I am confused with calculation of Zs. Using the formula Zs=Ze+(R1+R2) then a typical value for a type C 32A mcb to BS EN60898

would be 0.8ohms.For a TN-S supply then Zs=0.8+(R1+R2) so whatever the values of R1+R2 surely the installation will not comply?

Thanks in anticipation for any input. This is kind of work related. I work in luxury motorboat industry and we are having to come up

with a testing procedure(2x 3phase generators and 3phase shore supply with isolation transformer)

Thanks guys

 

[kme]

That is why using a C32 is complicated - you MUST be able to get the loop within tolerances.

If you have a TN-S install with a 0.8 Ze - you are going to have a bit of a problem - irrespective of the cable run.

You are working it out correctly - A C32 needs a very low earth loop!

KME

 

[st3v3]

I am taking 0.8 Ze as a theoretical value on the generator supply with a 32a supply to cabin consumer unit. I have no actual measured value just getting vessel ready

for power up!

 

[Andy™]

how big is the genny? its max current may not be enough to trip a C32 if there is a dead short.

also the loop of the genny, even in TNS, may well be over 0.8. the loop on my 3kW genny is around 4 ohm iirc

 

[st3v3]

We are using 2x40kw generators that have sync capabilities with the shore supply (dependant on load) I counted up 106 circuits -boat is 105 ft and just picked a 32a

cabin supply as an example.Wanted to make sure I am going down the right road with testing procedures and/or calculations.All circuits designed by our development

team but not too sure about their electrical qualifications. I am apprenticeship sparks but been in marine industry for 18 years.

 

[Andy™]

0.8 is the max the Ze should be for TNS. nothing to stop it being lower, which it would usually be. which then gives you a little more play with R1R2

 

[st3v3]

I was hoping to rely more on calculations from dead tests but I guess nothing beats actual measurements although there are lots to get through with

3 supplies! I was just having a bit of trouble with the math from Zs tables and the formula!