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Thread: Phase rotation and PSC

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    Full Member Master Electrician's Avatar
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    Phase rotation and PSC

    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.

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    mikilianis (25-Aug-10)

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    Diamond Member AndyD's Avatar
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    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.
    Last edited by AndyD; 25-Aug-10 at 10:30 PM.
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    Dave A (26-Aug-10), mikilianis (26-Aug-10), wizman (26-Aug-10)

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    Site Caretaker Dave A's Avatar
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    Quote Originally Posted by Master Electrician View Post
    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.
    The trouble with opportunity is it normally comes dressed up as work.

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    mikilianis (26-Aug-10)

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    Does this apply to a 3 phase supply as well.

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    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..

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    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.

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    Diamond Member AndyD's Avatar
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    Quote Originally Posted by murdock View Post
    .........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?
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    Sorry should have said the calculation for the PSC

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    Diamond Member AndyD's Avatar
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    Quote Originally Posted by wizman View Post
    Sorry should have said the calculation for the PSC
    PSC calcs on an LV 3-phase network can get complicated very quickly. The fundamental principals are the same as for the basic single phase method above but I would honestly recommend that if you're not familiar with these types of calculations then seek assistance from someone who is. No offence intended but PSC calculations on LV single phase domestic circuits are one thing, with 3-phase and possibly commercial and industrial installations mistakes and oversights in this department can prove costly and even fatal. Please don't attempt anything outside of you skill set.

    If you want some light reading just for your own curiosity on 3-phase PSC you can start here.
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    I cant find that anywhere... phase rotation must be clockwise.. if it had to be there would only be one box to tick on the form? im just curious.. if it is in SANS 10142.. i should have seen it at least once..

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