Power measurement

I am going to a customer site to measure machine power, while it is in an idle state. It has a 3 phase & neutral supply(50Hz).
My plan is to use a clamp meter to measure current,
is it simply a case of P=IV for power or is there a need to multiply by sqrt(2). Also, do I need to measure the 3 phases and add the reults together to get the total power consumption.
Thanks.
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From the question- you really don't know what you are doing so I suggest that you get someone who does know even if it costs a bit. Your customer might be far happier if someone competent did the measurements. If you want real power measurements a clamp on ammeter won't do. You need to a) have a wattmeter or some form of phase measurement b) know what you are doing.
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Don Kelly snipped-for-privacy@shawcross.ca
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If you don't know, just don't reply. That way I won't have to read throught he drivel you type.
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Craig wrote:

Don answer was exactly right. And you still don't have a clue what you are doing. A starter might be looking up "power factor" in Wikipedia.
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Firstly,
I can't remember saying I did have a clue what I was doing.
Secondly,
I may not be a competent electrician, but I am a qualified one so, at least in the UK, people will just have to put up with cowboys like me doing electrical work and giving superstars like yourselves a bad name.
Regards.
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Well in the US, to be a 'qualified electrician' you would have to know that for a *balanced* three phase supply the total real power flow is = sqrt(3) * V* I * pf. Where pf is the power factor. Power factor can range from zero to unity based on two general things, the phase relationship between voltage and current, and the harmonic content of the current. Phase displacement happens with inductive loads (motors most common) or capacitive loads (rare). Harmonic content can be high if supplying non-linear loads (electronic power supplies most common).
So using a clamp on ammeter will give you one of these terms and you can easily get the voltage term with a simple voltmeter. Finding the power factor means you need a different type of instrument altogether. The most common instrument for this is a watt-meter. It combines all three terms (voltage, current, power factor) in one unit, eliminating a lot of the work in measuring power.
Now, if the load is *not* evenly balanced (quite common in office environments), then you have more trouble. You can measure the current in each phase individually along with its harmonic content and phase displacement. Sum each of the three measurements (without the sqrt(3) multiplier). But you also have to consider that if you are measuring voltage line-to-line, there is a 30 degree displacement from line-to-neutral that can skew your phase displacement measurements (instrument transformers used in permanent metering can complicate this even more).
Now, if you think the others are giving you a rough time about 'not knowing what you're doing', it's probably because nothing in your original post suggested you had any idea about any of these factors. If you did know this stuff, then you wouldn't have suggested just using P=VI and you would at *least* have known that its the sqrt(3) that gets tossed around in three-phase calculations, not sqrt(2).
daestrom
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Craig wrote:

Wow, just 4 days ago Sue/Palindrome said: The system in the UK is, admittedly, wierd. The actual person doing the wiring needs no qualifications at all, specific or unspecific. He could have been at school the previous week.
You dismissed Dons answer as drivel.
You apparently didnt take my suggestion and look up power factor in Wikipedia. If you did, you would have found a significant portion of what daestrom posted, and perhaps could have asked an intelligent question.
Somehow "superstars" like me have been given a bad name. "Superstars" like me include most licensed electricians in the US and about everyone that follows this newsgroup.
Your behavior is not conducive to getting responses.
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Unfortunately, I am not familiar with the legal position surrounding the UK system. All I know is I got a job as an apprentice electrician, had a combination of 4 years mixed work/study, sat an achievement measurement test (AM2), followed by a further 6 months work then was given a card by the JIB stating I was an electrician. Not sure if there is any other route and to be honest I don't particularly care.

It was! I was looking for a few pointers. The fact of the matter is, after a couple of years electrical work, I done a BSc in software development then a masters in networking. I work in machine integration, getting hardware and software together. It has been nearly fifteen years since I had to do any basic electrical theory and when my boss asked me to do this, I thought I would ask for suggestions before commiting. I wasn't looking to be told how clueless I was.

Stick to electrcal work, claivoyance clearly isn't your thing. I did in fact look at Wiki as suggested. Then I looked out my old college notes and found the information I was looking for. I am still unsure where you got the idea I hadn't looked up the reference you gave me, albeit after you told me.

After asking a fairly basic question, I didn't get a conducive response so what difference does my behaviour make?
I don't doubt the knowledge each person in this newsgroup has in this area, but please remember that out of your particular area of expertise, you may also ask questions that seem ridiculous.

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remove the X to answer ----------------------------

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If you don't know don't mess. Would you measure your gas supply?

Trained industrial electricians are hard to come by, people change a plug and think they know it all

Unqualified and incompetant what a combination

Obviously you have met house bashers and not industrial trained sparks

Yes you are within your ability to ask questions but not within your ability to understand the most basic fundamentals of electricity ie Ohms law ( yes it is spelt correctly) Might i suggest you get you copy of the Yellow Pages and look up a good industrial sparky and sub contract the job out at least this way you wont end up killing yourself or anyone else.
Gavin
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No, but I would measure somebody elses

Perhaps they ought to change their job title to something more fitting, a word of caution though GOD is already taken.

I am very qualified, but I don't dispute the incompetant part

I have never worked on a house, only industrial projects and most of the people I worked with were struggling alcoholics who could hardly string a sentence together. This in no way reflects on you personally, or the people you work with. I am not in a position to comment on that. I am, however in a position to comment on my personal experience and stand by what I have said.

Speed = Distance x Time. I understand perfectly

I don't need the yellow pages, all I need to do is go to the local boozer and I will find dozens there.

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Which course is that? and when did you take it?

Thats Newtons law Doh

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Power factor improvement & three phase theory (Module No 2160070) June 1994. Much of it was covered again in the HN unit Electrical principles(Module No 2400040) which I passed with merit in June 1995 as part of an HNC electrical/electronic engineering.

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So you have an HNC (electrical or electronic or other ?) and ask a basic question that you shoould already know the answer to. So much for HNC

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Yes
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----------------------------

Then why did you ask the stupid questions that you did? Did you remember anything about the course except the it's name? My original answer was not to put you down. You have done that to yourself.
Bye
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gavin wrote:
<snip>

The lesser-known one that he wrote after a *very* heavy apple hit him on the head, presumably?
--
Sue




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@privacy.net says...

Took daze to figure that one out.
--
Keith

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