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

 

[saspm]

I think he possibly means 50v/0.03 for 30 mA

 

[Ian1981]

Ian, sorry I'm confused, where did the 30A come from in your calculation? I may have missed something, hence my question

A 6amp type b mcb will trip instantaneously at around 3-5x rated current.
6amp x 5 is 30amps for type B.
That's where the 50v/ 30 comes in for that particular type of mcb.

 

[Ian1981]

A 6amp type b mcb will trip instantaneously at around 3-5x rated current.
6amp x 5 is 30amps for type B.
That's where the 50v/ 30 comes in for that particular type of mcb.

As the max zs is 7.28 ohms on a 6amp mcb type b it's 230x cmin 0.95=218.5volts /30amps=7.28 as this is max zs value for the mcb.

 

[Ian1981]

I think he possibly means 50v/0.03 for 30 mA

No that would be 1667 ohms which I've already explained earlier in the thread

 

[saspm]

No that would be 1667 ohms which I've already explained earlier in the thread

Yeah sorry! Just seen that

 

[Pete999]

Thanks thought I had missed something

 

[Deleted member 26818]

There are as Westward said, two parts to an MCB.
The magnetic and the thermal.
The thermal part and fuses work because they get hot.
Fuses get hot and melt.
MCBs have a bi metal strip which heats up and bends until it trips the switch mechanism.
The bi metal strip is made up of two metals which expand at different rates, as the strip heats up it bends towards the side which has the metal with the lowest expansion rate.
This is similar to an indicator relay in a car or motorbike.