Zs, Ze how the tester works?

assegayer · Mar 31, 2017

I understand how to test Ze and Zs and that Zs is the total impedance from the suppliers tranny to the final accessory in a circuit your testing. Also that Ze is the suppliers earth resistance.

However i have a few questions

1.) how does the test meter actually work to measure Ze by connecting the meter to the main earth (obviously main earth disconnected from MET) and to the phase conductor? does it link it out or send a current or what?

2.) Why and how does Zs effect a devices tripping time? like what is the theory behind it. I know Low Zs means that fault current will be higher thus tripping the device but how does it trip the device and what device MCB ? RCD? how does fault current going down earth trip a MCB?

3.) Would you be able to just do a test from the main isolator in the disboard and record that measurement for all the Zs readings (adding Ze reading off course) on different circuits or does it have to be done individually for each circuit to get the furthest point in the circuit?

4.)Does the meter measure prospective fault current by just using ohms law and dividing mains voltage by resistance of the cable but what i dont understand is how does the meter find the resistance of the cable does it pull a load on it and then calculate or what?

Thank you for your time

mickfred · Mar 31, 2017

A few points Zs and ze are different reading , if you test at the mains as you say that will be ze ,you could link out a light (l-e)and test at the board but this would be a r1+r2 test witch is done dead not live . Efli testers don't trip mcbs they can trip rcds ,

Leesparkykent · Mar 31, 2017

mickfred said:
A few points Zs and ze are different reading , if you test at the mains as you say that will be ze ,you could link out a light (l-e)and test at the board but this would be a r1+r2 test witch is done dead not live . Efli testers don't trip mcbs they can trip rcds ,

R1+R2. Big R not little r as this is end to ends.

telectrix · Mar 31, 2017

basically your meter can only measure one thing.i.e.current. depending on what setting you are using, this current is converted by the meter into volts or ohms . think about a dead resistance measurement. the meter applies a voltage to the circuit (from it's batteries) and measures the current. this is translated into ohms by the mater using ohms law through it's internal electronics. when measuring Ze and Zs, the voltage is derived from the supply, not from the meter. this is why a separate EFLI tester does not have batteries.

mickfred · Mar 31, 2017

There is ways round testing at the board on high current setting that bypasses the RCD tripping . Sure someone will be along soon who will post a diagram ( i cant seem to post pictures )

telectrix · Mar 31, 2017

here:

[ElectriciansForums.net] Zs, Ze how the tester works?

mickfred · Mar 31, 2017

Leesparkykent said:
R1+R2. Big R not little r as this is end to ends.

noted

westward10 · Mar 31, 2017

Mcbs work on thermal and magnetic principles. Magnetic for sudden rush of current like a short circuit which the thermal side will not sense quick enough. The thermal side is for overcurrent. The fault path will send current down the earth and line conductors and it is the line current which activates the device. Unless a device is faulty it is current only which activates a device or fuse.

mickfred · Mar 31, 2017

Off topic . I keep getting bombarded with Klein tools ads (the prices seem a bit steep ) has anyone who is not a yank ever brought any

Wilko · Mar 31, 2017

Hi - I'll do an easy one :
4 - yes, if the meter is connected to measure Zs and then we flick to pfc, it is just using the rms voltage and applying Ohm's Law to give pfc.

assegayer · Mar 31, 2017

Wilko said:
Hi - I'll do an easy one :
4 - yes, if the meter is connected to measure Zs and then we flick to pfc, it is just using the rms voltage and applying Ohm's Law to give pfc.

but why do you need to put a probe on the earth and the line conductor? does the meter short them out or something?

assegayer · Mar 31, 2017

I am still quite confused on why loop impedance has anything to do with how quick a protective device would trip? i just dont understand how earth has anything to do with a mcb tripping in short circuit conditions or the RCD tripping in earth fault conditions surely the RCD will see the difference in current regardless of the earth being high impedance or not?

assegayer · Mar 31, 2017

mickfred said:
A few points Zs and ze are different reading , if you test at the mains as you say that will be ze ,you could link out a light (l-e)and test at the board but this would be a r1+r2 test witch is done dead not live . Efli testers don't trip mcbs they can trip rcds ,

what confuses me is when i am always told Zs = Ze (R1+R2). so does that mean R1 and R2 is measured for each individual circuit by being cross connected then the ze measurement is added on that by the person testing to give Zs? so Zs is basically just R1 and R2 cross connected plus the Ze reading?

Its because i saw a video earlier showing a person testing Zs from a scoket outlet. but surely you can just test it like that because you would have to add the Ze reading?

Ian1981 · Mar 31, 2017

assegayer said:
what confuses me is when i am always told Zs = Ze (R1+R2). so does that mean R1 and R2 is measured for each individual circuit by being cross connected then the ze measurement is added on that by the tester to give Zs? so Zs is basically just R1 and R2 cross connected plus the Ze reading?

Meter sends a current 25amps through live and earth when meAsuring ze and records the volt drop and converts to ohms on the meter.

mickfred · Mar 31, 2017

Cross connection R1+R2 is for ring final only

Wilko · Mar 31, 2017

assegayer said:
but why do you need to put a probe on the earth and the line conductor? does the meter short them out or something?

Hope not, there'd be a Big Bang

If we want to measure the resistance between two points we need to apply the test to those two points. Zs is between line and protective conductor, so leads are attached to line and protective conductor. Check 411.4.5 for definition. In terms of the meter that takes the measurement, it is a bit special as the circuit is live and in service. Not your average multimeter set on low Ohms range

assegayer · Mar 31, 2017

Ian1981 said:
Meter sends a current 25amps through live and earth when meAsuring ze and records the volt drop and converts to ohms on the meter.

but how can it record the volt drop when your not at the other end of the cable to measure the volt drop!!! does it magically go around the live and back through the earth?

assegayer · Mar 31, 2017

Wilko said:
Hope not, there'd be a Big Bang
If we want to measure the resistance between two points we need to apply the test to those two points. Zs is between line and protective conductor, so leads are attached to line and protective conductor. Check 411.4.5 for definition. In terms of the meter that makes is measurement, it is a bit special as the circuit is live and in service while the meter is taking this measurement. Not your average multimeter set on low Ohms range

but this is the concept i am struggling with you cannot measure the resistance of one piece of wire unless you have long leads either end or unless of course you cross connect and measure at end point?! thats why i am wondering how the meter does it for Ze

mickfred · Mar 31, 2017

This may be a ---- take , could be wrong

assegayer · Mar 31, 2017

mickfred said:
This may be a **** take , could be wrong

honestly no ---- take mate but i like to know how things work so i understand completly what i am doing it makes the job tons easier especially when fault finding

westward10 · Mar 31, 2017

Understand the principles of circuit protection and don't lose too much sleep over the operation of an earth fault loop impedance tester, I commend you for wanting to know but it is clouding over what you NEED to know. Trust me it doesn't matter how it works.

mickfred · Mar 31, 2017

My mistake it just you use all the right terms so i thought you may be a spark yourself

Ian1981 · Mar 31, 2017

assegayer said:
but how can it record the volt drop when your not at the other end of the cable to measure the volt drop!!! does it magically go around the live and back through the earth?

In simple terms, the impedance of the phase-to-earth loop is measured by connecting a resistor (typically 10 Ohms) from the phase to the protective conductor as A fault current, usually something over 20 A, circulates in the fault loop, and the impedance of the loop is calculated within the instrument by dividing supply voltage by the value of this current. The resistance of the added resistor must be subtracted from this calculated value before the result is displayed. An alternative method is to measure the supply voltage both before and whilst the loop current is flowing. The difference is the volt drop in the loop due to the current, and loop impedance is calculated from voltage difference divided by current.

assegayer · Mar 31, 2017

mickfred said:
My mistake it just you use all the right terms so i thought you may be a spark yourself

I am almost a 'spark' mate am2 in june and done!

assegayer · Mar 31, 2017

Ian1981 said:
In simple terms, the impedance of the phase-to-earth loop is measured by connecting a resistor (typically 10 Ohms) from the phase to the protective conductor as A fault current, usually something over 20 A, circulates in the fault loop, and the impedance of the loop is calculated within the instrument by dividing supply voltage by the value of this current. The resistance of the added resistor must be subtracted from this calculated value before the result is displayed. An alternative method is to measure the supply voltage both before and whilst the loop current is flowing. The difference is the volt drop in the loop due to the current, and loop impedance is calculated from voltage difference divided by current.

THANK YOU!! now that has made a switch flick in my head, just a one more thing if you can how does Zs effect the tripping time of a MCB or RCD?

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