Ok, there is much to unwrap here – and I am struggling to do so!
Let’s try talking it through – why do we want the Zs - there are two reasons, and let’s apply them to a domestic situation then industrial, then generation.
Firstly
We need to find out the MINIMUM fault level so we can confirm the protection operates (this is the same at transmission, distribution and domestic levels – and everywhere in-between).
To do this we normally remove any paths which we cannot guarantee will always be there – such as in domestic where the Bonding connects the MET to the water pipes, which connects to the boiler, which is connected to the FCU and hence the board via the CPC of that circuit, or the supplementary/EP bonding in the bathroom connecting the radiator to the lighting circuit and the boiler, through the water to the MET etc etc.
The easiest way is to disconnect the CPC from the board for the circuit under test, and measure the R1 + R2, noting we must ensure in doing this that there is NO other connection to earth, then add it to the source impedance (usually measured as Re) to get Zs – providing this is low enough to operate the associated fuse, MCB, MCCB…. or whatever) we are golden.
In a domestic this is generally all we need to do.
The second reason.
…Is to determine the MAXIMUM fault level to ensure the equipment is able to interrupt the circuit.
In this case we actually need everything in place which may contribute to this level – usually this is just ensuring all parallel paths (such as the circuit CPC and bonding with all circuits connected etc), however when we deal with industrial and generation sites, this may not be sufficient.
We could just carry out R1+R2 tests (added to Re), but with everything remaining connected to ensure all paths are in place, or more simply Use a MFT to measure Zs directly again with all paths in place.
In domestic this isn’t usually necessary as firstly we have one source Re through one main fuse into a CU which is type tested to coordinate with the standard supply fuses at all likely fault levels, and secondly, we only need to measure at that one point (the CU).
In industrial however, this is often required as we cannot guarantee such coordination, so it does have to be measured, (and why industrial kit may be 3kA, 6kA, 10kA, 16kA…..), but also remote switchboards may actually have lower fault levels - for example the MCCBs in the main switchboards need to be 50kA capacity (including the feed to the remote switchboard), but the remote switchboard itself may only need 16kA MCBs, and another switchboard downstream only 10kA etc etc.
So, if we are looking for Zs and it’s for minimum fault level we ought to use Re + R1+R2, we shouldn’t really measure Zs directly.
But, if we are looking for Zs and it’s for maximum fault level we ought to measure Zs directly, and we shouldn’t really use Re + R1+R2.
One issue with industrial is that there is often a huge bank collectively of motors, both large and small, which when running actually act like generators when a fault is applied - usually this doesn’t impact the steady state fault level, but it does significantly impact the initial fault level.
This is why there is a difference between switchgear used in domestic and industrial – a 6kA domestic MCB would have to withstand something like 1.5 x its braking capacity (6kA = 9kA) over the first few milliseconds – which has generally fallen to 6kA by the time it actually opens. However, an industrial one due to the motor contribution may have to withstand 2.5 x its braking capacity over the first few milliseconds – which will not have actually fallen to its rated braking capacity by the time it actually opens - this is fine because the standards for industrial MCBs take this into account, and they are tested to confirm this.
This is actually worse on a generation site – you measure Re when it’s connected via a weak link to the national grid (and hence the generation), but when it’s running the local generation provides well more fault level than the grid would – the design must take this into account.
So all the drivel done with, in your case
- What are you measuring the Zs for – why does the Engineer want this – it’s likely to be for minimum, but it matters as to how you configure for your tests.
- You appear to be measuring Zs at a switchboard rather than Re – but with earths disconnected – if the earth is disconnected properly I would expect Zs = infinite, if it isn’t then the earth is not disconnected
- If this is an active site remote activities can impact your readings (you disconnected the earth, but remotely someone placed a spanner or something between metal clad switchgear and the metallic framework of the building – actually linking your “disconnected” earth to earth – many many reasons)
- When a MFT measures the Re (or Zs) directly, it does so by applying a number of direct short circuits – but of very brief durations, noting the spike in current and estimating the fault level - if this is done on a site with major contribution via motors or generators this could be totally un-representative of the steady state value – the reading could be wildly out (too high or too low) – A MFT effectively measures a very peak value, and back-estimates this to what normal steady state fault level would produce this sort of peak. If the X/R ratio of the location is significantly different than a “normal” X/R ratio such as directly on a transformer, or having local generation (true generators or motors) then the results can be spurious.
