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.
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.
I can't remember saying I did have a clue what I was doing.
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.
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).
Wow, just 4 days ago Sue/Pal?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 Don?s answer as ?drivel?.
You apparently didn?t 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.
I do know. You apparently don't. That is the reason for my comments.
If you are qualified- then there are problems with standards in the UK.
Qualified doesn't mean competent and, in your case the lack of competence
However, your "knowledge" and "competence" is, fortunately, not typical of
UK trained electricians that I have met. It appears the exception makes the
You are out of your depth. May I suggest that you try thinking instead of
Don Kelly firstname.lastname@example.org
remove the X to answer
Unfortunately, I am not familiar with the legal position surrounding the UK
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.
Obviously, or I wouldn't have asked
You may well be correct, but how can we compete with a nation that gave us
McDonalds, George Bush and Friendly fire
Again, I stated this in a previous post
I think I have met more UK trained electricians than you and most of them
can't tie their shoe laces
On the contrary, after reading through my old course notes etc, I am well
within my ability with regard to the original question asked.
Do you know what 3ph is? and i don't mean 415volts.
Do you know how it is produced?
Does your coustomer know that you are unqualified?
Where will you place the clamp?
Does it have star delta starter?
Capacitor start motor?
Is it VSD controlled?
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.
I was going to help until i saw the comment about UK sparks
Just clamp the cables and your wife will become a very rich widow as you
don't know what you are doing and have no understanding of the basics of
No, I am keeping it a secret, please don't tell them.
Around this green/yellow wire I have seen lying around.
No, initially it is wired star and when it is up to speed, I quickly put on
a pair of
rubber gloves and rewire it.
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.
Good- then show it.
By the way, I am not from the US and "friendly fire" was invented far
earlier -and not by the US- one example was when the RAF bombed Canadian
troops near Caen in WW2 because they misunderstood signals.
Bush and Macdonald's - I grant you those but I've had pretty bad fish and
chips in the UK.
As for the good UK electricians- how can you judge -given your admitted
The "drivel" was intended to show you the way- either get someone competent-
or learn more. If you have done that -good for you. A BSc in software is not
going to give you much information regarding polyphase measurements.
Yes- I could tell you what measurements should be made, why and how, but
since your background was not known when I replied- but your ignorance was-
it would not serve anyone, including you, and would be irresponsible of
me-like giving a loaded pistol to a baby. I thought of mentioning factors
such as root (3) and power factor but then considered that that was over
your head- given what knowledge that you had shown.
Don Kelly email@example.com
remove the X to answer
Did I mention that as well as being an incompetant electrician, I was no
good at history.
I am sure there are some very good, knowledgeable UK electricians,
my point is, the sparks I have worked with have spent the last 2/3 of their
lives bending conduit and making off the ends of wire armour. Like me, they
probably taught the theory at some point, but when you don't keep using
it is easily forgotten.
I wasn't trying to show how clever I was, merely point out that I had moved
into a different
area of work/study and offer this as a possible defence of my apparent
basic electrical engineering.
So you had two choices, not post anything or reply with an insulting post.
You chose the latter.
I accept some responsibility for this, I could have given a little more of
my background or at least done a quick
google for power factor and found my own course notes before posting.
However, I didn't expect to come
under the wrath of a spark, eager to show how clever he was with his ability
I spend most of my days dealing with IT engineers some of whom claim to have
engineering degrees but could not descern the difference between basket tray
and cable tray.
Theory is easily remembered if you try
I to do not deal day to day with installs anymore i have moved into several
different areas including consulting. I do not have a degree but City and
Guilds 2361; 2362; 2377; 2380; 2391 and 2400. I now run projects mainly
energy saving initives and specialist computer room projects
The attitude you show is outstanding arragrant and ignorant you would make a
fine traffic warden / solicitor / politician
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
which I passed with merit in June 1995 as part of an HNC