Earth loop impedence – Ze reading

Earth loop impedence – Ze reading

When testing an electrical installation the first test I like to perform is an Earth loop impedence test, sounds complicated but dont let the big words put you off.

This is basically a test performed to ensure the Earth supplied from the electricity supplier, for example in your house is a good sound Earth, is of low electrical resistance and can be proven to be so.

To first understand this, there are Three ways in which the electric company brings an earth connection to your property.  There are more commonly just two, if your property is relativly new then it will probably be a TNC-S supply, if it is older it will more than likely be a TNS supply.  The third less commonly used is mostly found at remote locations like farms and raural areas & is referred as the TT system.

Pictorial examples to simplify

The below is a typical TNC-S supply, this uses the Neutral conductor as a shared Earth, the reason for this is most newer cables are either not covered in Lead or steel armouring and only have Two conductors in the cable


Below is an example of the TN-S earthing system, this is found in older dwellings and the Lead covering in the cable is used as an Earth connection.

You can normally see which is which as the older TN-S system has the Earth wire soldered on to the side of the main incoming cable, or may have an Earth clamp attached with the Earth cable affixed by a screw terminal.

The TT system

This is not common in towns & cities as this system is mainly used on pole routes to remote locations such as farms or raural locations.

In this system there is no earth supplied along the route and the means of earth is usually a copper rod driven into the ground in close proximity to the mains intake to a dwelling or building

I will cover this in a future post as the set up is a little more complex & there are additional requirements needed to comply with the electricity regulations BS7671.  As it is less common the chances are you will probably not encounter this system and I have only ever dealt with less than a handfull over the last 25 years or so.

Earth path back to generating station

As you can see in the above diagram the Earth goes back to the electricity generating station, this is the same for both cases (TNC-S & TNS)

If you look at the centre point both the Neutral & the Earth are at the same potential which is why the TNC-S can use the Neutral as an Earth.  There are drawback in that the longer the cable the more resistance there is, to counter this the electric company will use earth rods along the length of the Neutral to ensure a good earth with low resistance is maintained, without the earth rods there would be a high resistance in the earth path & this is not what we are after, the lower resistance the better the earth connection.

The diagram above also shows the fault path if a LIVE conductor makes contact with an earthed chassis or housing, it is the current that flows down this earth path that trips the breaker or blows the fuse, we are after the highest fault current possible this is why the earth path needs to be at the lowest resitance to allow this fault current to flow back to the generating stations earth connection as quickly as possible, for this we need a high fault current.

If the earth path has a high resistance then the fault current will be too low and may not operate the breaker or blow the fuse, so you can now appreciate why the earth is so important in any electrical installation.

The term earth fault loop impedence has a short abbreviation known as ZE, each type of earthing system has a maximum value of resistance that deems it either a good earth path or a bad or unacceptable earth path.

ZE is measured in Ohms, the unit of measurment for resistance, to take these readings you will need a Earth loop tester or most multifunction testers if not all will have this facility.

This is a LIVE test so extreme caution is needed when undertaking this test, the test is relativly simple & quick and gives you a reading in Ohms of the earth path.

For a TNC-S the reading should be under 0.35 Ohms to be deemed as good, for the TN-S the reading should be under 0.80 Ohms to be deemed as acceptable.

Performing the ZE test

To undertake this test you should really be trained on how to do so or have someone who is quallified & compitent to instruct this procedure as this is a LIVE test and there is risk attached to undertaking this.  As there is no protection offered to the person undertaking this (by this I mean no means of automatic supply disconnection) then if in any doubt DO NOT DO THIS !

To perform the test the main earth needs to be removed from the installation, the reason for this is to eliminate any parallel earth paths as this will give a false reading and could hide any potential earth issues, which really negates the purpose of the test as we are interested in knowing how good the electrical supply companies earth is.

Parallel path will exist in a normal installation as the water & gas pipes will be connected to the suppliers earth on the main earth terminal (MET) and this will all effect the reading as these paths will lower the earth resistance reading if they are not removed first

Once the Earth has been removed from the main earth terminal the test is measured between the incoming LIVE and the suppliers earth cable, this will then give you a reading in Ohms, there are Two other readings that most test instruments will display these are PFC & PSC, I will cover these in a seperate post.




Additional reading to assist in ZE testing

There are lots of books available that cover Ze testing & one I can reccomend it the IET Guidance Notes 3 book, this covers External Earth Loop Impedence Testing in some detail as this forms part of the C&G 2391 testing & inspection criterion and is a must for anyone wishing to undertake this examination.

Please note

If you are in any doubt about undertaking any electrical testing, then dont ! I cant stress this enough.

If you are interested in electrical testing then training is of parmamount importance as is gaining experience, for this I can highly reccomend NAPIT who offer affordable, reliable and very informative training packages delivered by trainers with years of practical experience and a mindfull of knowledge.


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