Harmonics

steve100 · Nov 30, 2012

We had the following question for homework the other day and I think the answer is 'C' (reg 523.6.3) but my mate thinks its 'A', does anybody k ow the correct answer?

In a TP & N installation servicing an office with IT equipment and a high harmonic disturbance. How would the harmonic influence be combated?

A. Surge protection
B. Insulation of live parts
C. Increase the size of the neutral
D. Install an RCD

Thx

Marvo · Nov 30, 2012

Your mate is wrong he owes you at least a beer.

Harmonic influence would include elevated neutral currents especially if the harmonics are in the TripleN range (3RD, 9TH, 15TH etc). Switch mode power supplies which are found in PC's, phone chargers, laptop power supplies etc are infamous for creating harmonics which can result in overloaded neutrals especially further up the supply network. It's become such a larg problem that there's new tighter legislation being introduced in Europe regarding the allowable THD and minimum power factor of these power supplies.

Paul.M · Dec 1, 2012

Fantastic answer Marvo.

You dont need to know this but...... In the not so distant future on the 3 phase side of things, there will be regs for the increased CSA of the neutral for the above reasons. At this moment in time concentrate on the the regs of the time.

Marvo · Dec 1, 2012

It's always good to know planned future developments especially being so far out of the loop in Africa. I can't believe they've let the manufacturers get away with it for so long, the only reason it's an issue is because of cheap manufacturing and sloppy design. They should have legislated more stringent minimum standards at least a decade ago then they wouldn't need to be increasing the load carrying capacity of the neutrals in the distribution network.

steve100 · Dec 2, 2012

Will defo call in the beer now!!

I have another query, we had another question in relation to the IR test. It was my understanding that when testing an installation as a whole, all fuses in place and circuit breakers closed (so testing all circuits at the same time), that the IR test reading should be greater that 1M ohms (according to the regs). However the answer was 2M ohms, which I understand that the OSG guide recommend that if the reading is above above 1M, but below 2M ohms then further investigation is required. So what is the correct answer to the question? Would it be 1M ohm if testing the circuits individually and 2M ohms if testing as a whole..... although the regs only mention 1M ohm??

Paul.M · Dec 2, 2012

steve100 said:
Will defo call in the beer now!!

I have another query, we had another question in relation to the IR test. It was my understanding that when testing an installation as a whole, all fuses in place and circuit breakers closed (so testing all circuits at the same time), that the IR test reading should be greater that 1M ohms (according to the regs). However the answer was 2M ohms, which I understand that the OSG guide recommend that if the reading is above above 1M, but below 2M ohms then further investigation is required. So what is the correct answer to the question? Would it be 1M ohm if testing the circuits individually and 2M ohms if testing as a whole..... although the regs only mention 1M ohm??

Hahahahahaha I was confused by this as well my friend and it did my head in. Let me explain...

A "global IR test" checks every piece of cable (L to N, L to E , N to cpc or L+n to cpc) of the full circuits of the install. If that result gives you a reading of less than 2 MOhm you need to check each circuit to find the weak point. Now if the weak circuit that you test shows you a reading of (lets say) 1.5 MOhm it has passed!

This test is a quick and easy way of doing it and not spending time per circuit (but I always do per circuit). On a new install that you have done, it is a time saver if the global test proves that the circuits are above 2 MOhms but if I got a result of lets say 3.4 MOhms I would test each circuit. It is how you interpret the MFT's results. As a tester its down to you.