Zs, Ze how the tester works?

[assegayer]

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

 

[Wilko]

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]

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]

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]

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

 

[assegayer]

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]

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]

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

 

[Ian1981]

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]

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]

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?

 

[Wilko]

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

Ahh - the missing piece for you may be - the line and the protective conductors ARE linked by the supplying transformer secondary winding. That is the circuit you are measuring. Does that help?

 

[assegayer]

Ahh - the missing piece for you may be - the line and the protective conductors ARE linked by the supplying transformer secondary winding. That is the circuit you are measuring. Does that help?

THANKS! believe me that was a missing piece mate now it really is coming together its just sometimes at college they dont clarify things like this and it keeps you wondering. Just thinking of it now that piece of information you just said clarifys alot of other things. Thanks alot

 

[mickfred]

Look in your post you have asked why a efli tester would trip a rcd , that each circuit is cross connectied for R1+R2 test and that you seen a video of some one pluging a tester in and if you add ze on to that value . That's why i thought you where either a diyer who has read this forum and picked up some of the terms or was a spark taking the Mick but if your doing you AM2 soon best of luck

 

[assegayer]

Look in your post you have asked why a efli tester would trip a rcd , that each circuit is cross connectied for R1+R2 test and that you seen a video of some one pluging a tester in and if you add ze on to that value . That's why i thought you where either a diyer who has read this forum and picked up some of the terms or was a spark taking the Mick but if your doing you AM2 soon best of luck

No problem mate thanks for the info anyway. I think there may be a misunderstanding i was asking why Zs has anything to do with how fast a RCD trips in fault conditions

 

[Wilko]

THANKS! believe me that was a missing piece mate now it really is coming together its just sometimes at college they dont clarify things like this and it keeps you wondering. Just thinking of it now that piece of information you just said clarifys alot of other things. Thanks alot

No worries, I've had plenty of wtf moments myself