Question regarding high Zs on 3-phase motor circuit using BS88 fuses

[Steveb555]

Industrial installation (non-domestic). Here's the scenario:

A large (150kW) 3-phase motor circuit where the supply is taken from a starter fitted with 315A BS88 HRC fuses.
From calculation using the cable data, the Zs value exceeds the manufacturers requirement for 5s disconnection time (similar to table 41.4 of the 17th Edition).
An additional earth cable has been installed to a main earth tape located near the motor end to achieve supplementary bonding.

My questions are: 1). How can this addition of supplementary bonding prove compliance with the regs? and 2). What use is an earth loop impedance test taken at the starter?

Any comments gratefully received.

Thanks in advance. Steveb555

 

[malcolmsanford]

What your doing by your supplementary equipotential bonding is trying to limit the touch voltages to 50v, so therefore if you have a high Zs by bonding your applying additional protection.

Where and what are you bonding. You can only bond an extraneous conductive part, so therefore sticking a load of G/Y cables to say a motor frame will not do al ot if it is not either an exposed/extraneous conductive part. You can check if this metal is earthy by doing the test as in 415.2.2

I'm not sure what you mean by your other point sorry.

 

[Steveb555]

Thanks for your response Malcolm. Much appreciated.
I can see your point of limiting the touch voltage so presume this should be tested as part of the regular inspections for installations which can't satisfy the 5s disconnection time by the Zs value?

My other point was just a question about the validity of the results from performing an earth loop impedance test because the result would not be influenced by any supplementary bonding at the motor end.