measure ment of 3 phase power

I have a doubt in electrical engineering.This is regarding 3 phase.How can i measure from which phase power is coming in a 3 phase connection

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It's the red one, because everyone knows red goes faster and gets there first.

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It will come from all three phases. You have voltage and current in each phase. It doesn't come from one phase at a time and normally you measure average power rather than instantaneous.

If you want to measure power in any one phase then your wattmeter must be connected to phase voltage and phase current. What is called phase voltage and phase current depends on whether the load is Y or delta.

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Don Kelly

I can only think that your question is aimed at determining, after a spider box, which phase an outlet is running off of. If this is so, turn off the box and unplug a single phase completely. Turn it on, and if it no longer works, you got your phase. If that's not the question, then I'll just give you some info on electricity to help with whatever you are doing...

Power is sent through three conductors from the power plant and is stepped down at a series of delta transformers until it gets to the entrance of a building or a neighborhood. At this location, a couple of things could happen:

  1. A delta transformer steps down to 240vRMS (US) with a center tap per phase neutral, grounded at that location. This allows for either two 120v hots relative to neutral, or 240v relative to each other.
  2. A Wye transformer steps down to 120vRMS (US) and generates a common neutral at the center, grounded at that location.

Once this is done, an added safety ground, grounded away from the neutral, is installed, and the system becomes either five or ten wire (the delta has three feeds per phase plus ground). Usually the delta method will then distribute the feeds so that they enter the home or office as either 3 or 4 wire (depends on if the ground is local or remote). Only one phase is usually sent to each, with a total of 240v possible from the two hots off of one phase (they are actually phase inverted from each other). These systems should NEVER be used with another phase. If the wye was used, as is common in situations where 3-phase motors are used (some central air, some industrial, all heavy rigging) or in entertainment venues with heavy power needs, each phase, the neutral, and the ground are sent to a disconnect where they can either be permanently wired into a device or can be left for manual tie-in.

Regardless, all power is measured RMS. 120v AC is actually about 170v peak. The RMS measurement between two phases is 207v. The RMS between a phase hot and its >I have a doubt in electrical engineering.This is regarding 3 phase.How

Reply to
Brandon Anderson

---------- One question - where do you live? In most parts of the world including the US, transformers used for 3 phase are mostly star delta. There are operational and economic advantages to this with respect to delta delta. For example, any appreciable length of overhead line fed or connected only to delta windings, can be trouble (this was found by experience, back in the

1920's). -- Don Kelly remove the X to answer


At this location, a couple

Reply to
Don Kelly


I tried to understand you actual questions (sorry). Did you means that what is the sources of three phase or how to measure the power between phases? For your information, the phase system could be in star or delta connections depend on user or system to use. To calculate the power from star or delta connections, you should understand the concept and formulas to apply with.



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Normally, a device designed to operate from 3 phase draws power from all

3 phases, and in approx. equal amounts. But if you have something like a 3 phase instant-on hot water system with 3 separate heating elements, one or two of the elements can burn out and this unequal load makes the power per phase unequal. To determine which element is burnt out you could remove one fuse at a time at the switchboard and see whether there is one of those fuses which has no effect on the heater's operation. If so, then you have identified that phase which is delivering no power. If you can obtain an AC ammeter of the split-core induction type which clips around any current- carrying conductor, then taking each phase in turn it will tell you the levels of current flowing in each of your three conductors while the appliance is in operation.

In the above I speak of CURRENT. To measure actual POWER you would need to connect a power measuring meter into each phase.

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John Savage

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