Can Zs at DB ever be less than the Zs of the feeding circuit?

Peter S3:

Zs at DB should be the value of Ze plus the total R1+R2 of the total circuit feeding the DB right?  R2 might be reduced at a particular DB by local bonding, but there will always be some R1 which will generally act to increase the total Zs at DB as you add more DBs into the chain?

See mapj1's posting above - the 4th response..


I think that you are correct.


I always think that it helps to draw out the installation.


My algebra isn't what it used to be, but call the impedance of the conductors to the final DB R1a and R2a; and the impedance of the final circuit conductors R1b and R2b. Zs db = Ze + R1a + R2a. Zs at let's say a luminaire = Ze + R1a + R2a + R1b + R2b. Bolt that luminaire to the steel frame of a building which is bonded to the MET. Now assume that the parallel path through the building has a trivial impedance so R2a and R2b may be discounted. Zs now = Ze + R1a + R1b and Zs db = Ze + R1a + R2a. If Zs < Zs db, Ze + R1a + R1b < Ze + R1a + R2a i.e., cancelling the terms which are on both sides, R1b < R2a.


So Zs of the final circuit will be lower than Zs db only if the parallel pathway has a trivial impedance and the impedance of the line conductor of the final circuit is less than that of the CPC of the distribution circuit. That would be most unusual!


I hope that this makes sense - I could have done better 40 years ago!


 

Peter S3:
@MHRestorations: that is entirely possible- we note that the Cal date for his test meter was 2 days AFTER the date of the inspection...

I think that annual independant calibration is not the best way if used solely.

lf A robust frequent checking even with something you built yourself is a good start.

Before Calcard I built up a resistance box for a few continuity and ins res checks.

A few sockets for loops and two RCD sockets for RCD tests (2 in case one failed).

I run all the tests monthly or more frequently.

I do not claim any great actual accuracy but it makes drifting easier to spot over a short period between such tests and a longer period overall. Not foolproof but has some benefit.

 

Peter S3:
@MHRestorations: that is entirely possible- we note that the Cal date for his test meter was 2 days AFTER the date of the inspection...

Don't consider that an issue, if you are running checks on your equipment throughout the year the period between calibration can extended for several years.


I was told to get my testers calibrated after three years as a non-compliance and was muttering about it as I stood at the counter in my wholesalers as I was waiting to be served and drop them off for the calibration. There were comments that you must get them done every year from other electricians, my comment was if you cannot test your own testers and be confident that they are reasonably accurate you should not be out and about doing electrical testing with your meters. 


If you can test electrical installations then you are capable of running interim tests on your own testers.


It should also be noted that using the same loop meter to measure in different ways 2-wire Hi, 2-wire Low and 3-wire Low at the same point will give different test results, you may get a different result at a different time with the same tester using the same method and all of them will usually vary from Ze+ R1+R2. Using different loop tester will return a different set of results altogether. The should not be huge differences, but things like having a RCD in circuit can add significantly to the test result.


I did the inspection and testing course along with the exam twenty odd years ago, digital meters were still relatively new products, my tutor passed comment that older electricians often went into melt down using digital meters. The older electricians would test using an analogue meter starting by twiddling the knob to zero it, then connect it and press test the button which sent the needle across the scale, they would try to look at the needle straight on to avoid parallax then take a reading as it wobbled about a bit, before announcing "That's okay" and writing a test result down.


Now with digital meters you get a result taken as a sample recorded to two decimal places resulting in electricians going into meltdown reaching for tables, calculators and books to confirm that "it's a good result" when all you really want to know is if the fuse will blow or the trip will trip. So if the maximum Zs for a fuse or trip is 1.1 ohms and you have a test result much lower than that it's job done and time to move onto the next thing, unless you get what is obviously a high reading for a large or short cable, in which case the terminals may need checking or something similar.


At times loop testing requires a bit of thought as to how and why you ended up with the result you got, also if it is an issue or not.


There is an old saying "Learn to test, test to learn".


Andy Betteridge