ZS Testing – whats it all about

ZS Testing (simplified approach)

In one of my previous posts I discussed Ze & the testing & measuring of Ze (external earth loop resistance) Zs is simply testing everything past the consumer unit or distribution board for each circuit.  There is more than One way of determining the Zs of a circuit, and being old I prefer the long winded but more simplistic approach rather than the quick method, I shall explain why shortly.

The quick Approach

Most if not all multi function testers will read straight Zs, to simplify this most manufacturers provide a three lead accessory that plugs straight into the tester & the other end has an appropriate plug on the other end for connecting directly into a socket outlet.  You then simply select Zs, press a button and the measurement is delivered instantly on the LCD display in Ohms.


You might think this is easy, yes it is, do I like it, No, No I don’t !  Why don’t I like this method as it is so quick & so easy, quite simply put it tells me very little, yes it gives me a reading and one that I can check off against a maximum Zs chart, if the reading is under the maximum allowed then the circuit passes the test.

What if the reading is just under the maximum allowed value say by 0.1 of an Ohm, would I be happy to pass this, the simple answer from me would be No, many do and as its under the maximum value it gets a pass.  Most electrical companies that perform EICR (electrical installation condition report) inspections & reports are happy to use this method and simply to report any reading over the maximum value because it is simple and fast allowing more circuits to be tested over time & they are not there to find or rectify faults, simply just to record & report.

If like me you are the One who has to find & fix defects as a result of EICR inspections then I would recommend my approach below.

My approach (R1 + R2 measurement)

The mathematical formulae for ZS is as follows Zs = Ze + (R1+R2), you maybe now thinking I know the Ze value (in Ohms) now how do I get the (R1+R2) value, also measured & recorded in Ohms.

To obtain an R1+R2 reading isn’t really that difficult, it takes longer to get this reading as there is a little more work to do to allow you to get it.  To keep it as simple as possible & in its most basic terms if you can imagine a shower circuit, One length of twin & earth cable, ignoring the double pole isolator, a circuit breaker at the consumer unit end & the shower at the other.

To obtain an R1+R2 reading firstly disconnect the supply in the consumer unit or distribution board, remove the Live conductor from the circuit breaker or fuse and connect this to the main earth terminal.  Go to the shower, remove the cover to expose the termination block and measure the resistance between the Live & the Earth terminals, the reading obtained is now known as the R1+R2 resistance.

If this reading is quite high as determined by looking at the maximum Zs tables (can be found in BS7671 the 18th edition of the wiring regulations book) then you need to check all terminations along the cable run.

In a shower or cooker circuit the most common place to check is in any double pole isolators or cooker termination boxes that should be fitted.  These are likely areas where terminals may be loose or burned or burning out, this will give a higher resistance reading than normal and is always a good idea to check terminations in such locations as a matter of good practice.

The diagram below is demonstrating an R1+R2 measurement but is on a lighting circuit, again if high readings are obtained then the usual place to look is in the light switch as these terminals very often work loose & give a higher reading than expected.

In the above diagram they have used a link wire from the Live terminal on the circuit breaker and connected this to the Earth termination block, either way is fine just as long as you ensure the supply is OFF beforehand.  As a matter of safe working practice, isolate the main supply into the consumer unit or distribution board before undertaking any testing operations.

Personal experiences of Zs testing

Over the years I have undertaken a multitude of EICR reports & inspections, and from very early on have always used the above method for obtaining Zs, in fact I don’t even know where the lead is with the plug on as I never undertake Zs testing using the quick method.

I have found all manner of faults due to higher than expected R1+R2 readings, none of which I would have even looked for if I used the quick & easy approach, as you can now see just because a circuit may have passed a Zs test & in most cases, just, there may be underlying issues such as burned or loose connections that may result in arching & burning so by obtaining the R1+R2 value can tell you a lot more than just a straight Zs reading.

Final Words of Caution

Please please always turn off the supply before opening up any electrical distribution boards, consumer units or any other part of an electrical system, this stuff burns if you’re lucky & kills if you’re not that lucky.

Work safe, Work smart, Go home alive !

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