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# How/Why do we take Zs readings

Discuss How/Why do we take Zs readings in the Electrical Forum area at ElectriciansForums.net

#### LukeScotty

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Welcome to ElectriciansForums.net - The American Electrical Advice Forum

Hello, everybody back at work today?? great isn't it!!

Could someone explain this to me please, i'd be rather greatful, cheers

: Reason for taking a Zs reading, what are you finding out, n what for, comparing it with other reading?
: Method : (i'm sure there are a few methods, depending on what machine) do you have to disconnect the bonding ??

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#### TonyM58

Ok Luke,

the Zs repesents the earth fault loop impedence. It consists of Ze (the external earth fault loop impedence) + (R1 +R2). R1 is the phase and R2 is the CPC (sorry if you are now sucking on an egg!)

So you are actually measuring the entire fault path from the supply transformer all the way to the house, through the dis board, down the phase to end of the circuit, then back along the CPC and incoming Earth cable to the supply transformer again.

Essentially what you are doing with the meter is strapping a resistor (about 10 ohms) between phase and earth (like a fault), which will cause a fault current to flow. The meter then measures this fault current and calculates the earth fault looop impedence from it, so it will display a value in ohms.

So lets say it measures a value of 1 ohm, then using ohms law I= V/R (actually Ia = Uo/Zs but its the same ting) , so I = 230 / 1 so a fault current of 230A would flow. If the Zs was 2 ohms, then only 115 amps would flow.

Now, take a Type B MCB (BS EN 60898), this has a an a trip current of 3 to 5 times In (its nominal setting). So a 6A breaker would trip at between 18-30A. Now the higher the fault current then the quicker it blow. And as we have just seen, the value of fault curent wil depend on the Zs. So the lower the Zs, the higher the fault current and the quicker the MCB will blow.

If the Zs is too high, not enough fault current can flow and the MCB may not trip at all.

Sockets, for example have to discconect withing 0.4 seconds of a fault, so take the (typically) 32A MCB they are protected by, this will have a maximum Zs value to allow enough fault current to flow to discoonect within this time.

in the OSG, if your circuits comply with table 7.1, then you just flip to the table of Zs values for your 60898 MCB, and read off the Max Zs value directly. If you are using BS7671 then you take the 'rule of thumb' and use only 3/4 of the value on the table plus further possible adjustments for ambient temperature.

For taking the Zs reading think of the worst possible scenario for the fault occuring - its at the end of the circuit where the resistance is highest, so this is where you measure Zs. if you have done your R1 R2 readings (which you will have!) then take the Zs at the point where your R1 R2 is highest. With test meters you will either just coonect to phase, neutral and earth, or just to phase and earth if its a two lead instrument.

Be aware the fault current you are introducing is EXACTLy what the RCD was put there to detect, so it will trip. Modern meters now have a 'no trip loop' setting (or something like that) so use that (with the 17th edition most domestic circuits wil be on rcd's anyway!)

You dont need to disconnect anything, the circuit should be exactly the same as when the fault is goning to occur (some books reccomend disconnecting the earth when measuring Ze)

there is an argument (of which i agree) that Zs values will be largely redundant for domestic circuits with the 17th, because the RCD will always trip long before the MCB

Hope this has been of help!

#### LukeScotty

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This has been a massive help to me tony, its helped me understand it alot better, I want to thankyou for typing that all out, v much appreciated.

Regards Luke

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#### TonyM58

No problems, its good to share knowledge, its always just difficult when someone asks because I have to judge their level of existing knowledge, so i can be informative - without being patronising!

Anything else, just ask away and i will do my best- i have a pretty broad range and if you are going industrial commercial i can talk about PLc's and invertors/soft starters/ machine controllers, as well as inustrial robotics.

I'll do my bet

cheers Luke

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#### Cirrus

he he think I might be plagueing you later in the year when I sit my 2391 Tony

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#### TonyM58

No problem Cirrus, how are you sitting it? Are you going 'slow-time' through a college or you doing a weeks intensive like I teach?

People make a big deal out of the 2391 and get wound up about it - but if you are able to test and inspect, then R1 R2 doesnt change, Zs is still Zs. Most people that get a problem get it with the written exam (averaging about 42% pass rate). But as I always tell my students, if you sit the course and pass the practical, then if you flunk the written exam you just resit it next time. If you fail your driving test you dont start learning to drive again.

Do LOTS of practice exam papers, and try to sit them under exam conditions i.e. closed book, and within the required time.

Most people fail because they simply havent prepared enough for the exam.

good luck

W

#### withers25

I would like to say this is a brilliant description I'm doing level 3 C&G long courses i have been on but working hard to get there !!

great to see so much help on here :waving:

#### darkwood

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I would like to say this is a brilliant description I'm doing level 3 C&G long courses i have been on but working hard to get there !!

great to see so much help on here :waving:

Thankyou for your contribution to the thread, check the posting dates if you can as this is a 8yrs old thread and on that note I think its time for it to close.

Feel free to start a new thread on the same subject

#### Marvo

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I would like to say this is a brilliant description I'm doing level 3 C&G long courses i have been on but working hard to get there !!

great to see so much help on here :waving:
We also have a trainee section on the forum which has some great info in it and there's experienced guys on hand who are committed to assisting the trainees. Check here for info on how to join.

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