Phase rotation and PSC

[Master Electrician]

Hi guys,
On the revered CoC, there are some check boxes that bother me:

1. Phase rotation. In the Cape Town area, the phase rotation is ANTI-CLOCKWISE. What is it like in the rest of the country, or are us Capetonians the only ones who have our wires crossed?

2. Prospective short circuit current in kA. How determined?
a) Calculated (Maths is not my strong point)
b) Measured (I am aware there are commercially available instruments that measure this - no maths needed)
c) From Supplier (provided you can get hold of an informed person)

I have not filled in a CoC for years, as I mostly do inspections on existing installations in hazardous areas.

[AndyD]

We were taught a method for determining psc of a circuit in a real life application. It requires only an accurate digital volt/ammeter and basic mathematics.

Firstly measure the volt drop by measuring the no-load voltage and subtracting the on-load voltage from it. Measure the on-load current in Amps and divide the volt drop by the current to give you a circuit resistance. Finally take the circuit supply voltage and divide it by the resistance you've just calculated....voila you get your PSC in Amps.

It sounds harder than it actually is, say you have a socket circuit with a no-load supply of 225volts. You plug in a 12 amp tumble drier and the circuit voltage drops to 223.3volts whilst it's running.

225v minus 223.3v gives you a volt drop of 1.7 volts
Divide the volt drop by the current - 1.7/12= 0.14167ohms
Finally take the supply voltage and divide it by the resistance - 225v / 0.14167ohms = 1588.2Amps.
Your real life PSC figure for that particular circuit is 1588.2Amps. Under a complete short circuit this is the maximum current that can flow momentarily before the breaker trips.

You can use the same method to calculate PSC from figures obtained by looking up the cable resistance and estimating the cable length etc but this won't take into account any extra circuit resistance from screw terminations or corrosion on connections etc.

The most accurate method however is by using a PSC tester. This tester actually uses a high current pulse to more accurately determine circuit resistance (usually in excess of 40Amps). The higher the test current is, the more accurate the results will be. There's obviously a limit to the test current so permanent damage doesn't result to the circuit being tested, also multiple tests to a circuit in a short time frame should be avoided as well.

[wizman]

Does this apply to a 3 phase supply as well.

[Dave A]

1. Phase rotation. In the Cape Town area, the phase rotation is ANTI-CLOCKWISE. What is it like in the rest of the country, or are us Capetonians the only ones who have our wires crossed?

I've just checked with my IE as to what we are finding in Durban. Apparently we're finding some installations on clockwise and others on anti-clockwise.

[Jacechop]

Does it matter? There is no 'standard' rotation.. So long as the rotation is consistant throughout the installation? Or rather until POC? A trick I heard from an old hand was to tick the box from supplier next to the kA rating..

[murdock]

phase rotation should always be clockwise when checking on a phase rotation meter...but if you have a fluke 435 it will be set to show anticlockwise on the display.

most of the machinery i work on must be connected clockwise...otherwise it will damage parts of the machine...we talking 2 to 5 million rand machines...you break it you pay...the motors are checked with a phase rotation meter before connecting the cable direction is checked then only is the cable connected and switched on.

and yes the answer to your question all three phase installations...there is no direction on a single phase installation...single phase motor windings can change the rotation of the shaft on a single phase motor.

[AndyD]

.........the machinery i work on must be connected clockwise...otherwise it will damage parts of the machine...we talking 2 to 5 million rand machines...you break it you pay....

Surely if rotation is critical then protection would be built-in to prevent counter rotation. If it isn't then wouldn't you install this protection as part of the installation?

[mikilianis]

Hi You state "the motors are checked with a phase rotation meter before connecting the cable direction is checked then only is the cable connected and switched on." may I ask how would one check a motor with a phase rotation meter I know one can check the phase rotation I am not being sarcastic I genuinly havwe a desire to know as I to work on machines where ddirection is detremental.
Mike

[murdock]

most of the machines with on board computors will signal a problem if the phase direction is incorrect...the problem is with motors for conveyers and pumps...if you run the motor the wrong way on start up there are serious implications...always keep your phases clockwise or anti clockwise whatever suits you...but in saying that i have found most machines i have worked on use a clockwise direction...so i choose to make all my installations clockwise...some machines have 10 or 20 motors on it imagine changing trying to figue out what is going on after a few clowns have worked on it...throughout a system makes it good practise...and i must stress again sabs electrical codes are there as guide lines...installing equipment straight...keeping direction clockwise red/white/blue...ABC...UVW just makes life a whole lot easier...my problem is i dont just sign over COCs i actually work in the industry...i design build and automate electrical installation and control systems...so i have a set standard to work to...it makes my life a whole easier...in fact i believe as mentioned before...this COC is a money making racket unless improved to a point that it is as many pages as required to carry out a thorough inspection report it should be scrapped...at present it is tooo vague and too easy to miss interpret...i see it on a daily basis...people just fill in what they assume the readings should be...as far as i am concerned...no coc should be issued without a full printed out test result from a megger or fluke multi function tester...of every single test carried out...and the inspection report fault list should also be attached to the COC...i feel like making a copy of a COC filled in a month ago by a company which i feel is illegal...it doesnt have the persons accreditation numbers etc...it isnt filled in correctly...results written on the COC are not even correct on it and what so scary is the person is a well known company who has been around as long as i have...and now the fun begins because my customer took my advice and once the sale was approved and the coc was been issued...we can now take action before tha transfer occurs...so it has been put on hold until all these issue are resolved...leaving my customer in a far better position than if the transfer had already gone thru...he has now decided to get me to carry out an inspection report...and the beuty about it is we dont need lawyers...or AIAs or DOL ...or anyone just the customer to be satisfied with what he gona invest the rest of his life in...and if he he not happy with the result just walk away and find a place he is satisfied with.



just 2 of the many testers available...


http://www.aemc.com/products/pdf/2121.04.pdf

http://www.megger.com/us/products/Pr...17&Description

[murdock]

just a reminder when using a phase rotation meter or any meter for that matter...not for sensitive viewers and if you are a young up and coming electrician i wouldnt advise you to watch this is your wife or girlfriend is close by...not because there are any dirty pics just because it will cause her to stress when you head out in the morning to work...i noticed they have put the music clip back on because apparently it has a copy right.


i show this video to my staff or when new people start or sub contract and when i catch them getting tooo clever...just to remind them of the implications of their actions.

YouTube- Donnie's Accident