ACM3P 3-Phase AC Ammeters

maybe they just wrongly call split-phase 2-phase ? Though in some places each house get two out of three phases, which from the meters point of view would be the same as three phase with no load on one of the phases

unless there is some timing that rely on 120degree I don't see why it wouldn't work on split phase

Win is in the US and we use 120 volts as our commodity power for household appliances. You can't use two taps of the three phase power for that unles s using a particular type of delta transformer that center taps a coil. Bu t then it's still 240 volt split phase.

The point is each side of the 240 volt circuit must be 120 volts from neutr al/ground because that's what we use for a return. True three phase doesn' t need a neutral for power purposes.

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  Rick C. 

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Not familiar with that meter, but the description sounds like a single phase ammeter with a push-button make before break selector switch - unused CT's must be shorted and the meter connected before the short is removed. This is the usual arrangement on 3-phase generator monitoring, although I have only seen them with rotary selector switches. You could put the third CT on the neutral of split single phase service to read all three current carrying conductors if you wanted, that would allow you to measure the excessive neutral current which occurs with haversine loads which would otherwise be missed. Sounds like a deal if it works and the range suits your load.

At one time 2-phase power distribution was used, with the two phases in quadrature:

but this is long gone, and an open phase in 3-phase results in single phase, so I am not sure where 2-phase power measurements would be useful today.

d appliances. You can't use two taps of the three phase power for that unl ess using a particular type of delta transformer that center taps a coil. But then it's still 240 volt split phase.

tral/ground because that's what we use for a return. True three phase does n't need a neutral for power purposes.

you can use two taps of the three phase power, it'll be 120V phase to neutr al and 208V from phase to phase

I have three-phase like everyone else here, it would be rather inconvenient with out a neutral, having to run everything on 400V

old appliances. You can't use two taps of the three phase power for that u nless using a particular type of delta transformer that center taps a coil. But then it's still 240 volt split phase.

eutral/ground because that's what we use for a return. True three phase do esn't need a neutral for power purposes.

tral and 208V from phase to phase

Yep and that's not what we use here. It has to be 240 volts from phase to phase so we can run our ovens an furnaces.

I suppose we could run them from 208 volts, but we don't because we have 24

0 volt split phase handy. If we did use 208 volts everything would provide 25% less power.

I see that when charging my car on level 2 chargers a lot. They are at com mercial sites where they have 3 phase and they just give 208 volts to the c onnection rather than bother with 240 resulting in slower charge times.

nt with out a neutral, having to run everything on 400V

I'm talking about here where we use three phase as three phase, not as a di stribution that you can pick lines off of for low voltage in the home. We have three phase on the power distribution, it's not very often brought dow n the street and hardly ever brought to a home. Pretty much commercial onl y.

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Oops, the haversine induced excess neutral current is strictly a three phase issue, not applicable to 120/240 single phase. I tend to think in

3-phase since 120/208 was used in every facility I have ever been paid to work on/in. 120/240 single phase is only found in residential and very small commercial facilities in the US.

This give me a chance to make a current-transformer rant. IMHO all CTs should always have a connected load resistor. And if that's not practical, they should have permanently attached back-back diodes, to save the transformer (the diodes won't conduct at normal FS load-resistor voltages). There's no excuse for burning out a CT. At this point in my CR4 rant, I posted pictures of huge destroyed, very expensive high-current CTs (too easily found with Google).

--
 Thanks, 
    - Win

What is split phase? Is that simply a single phase primary and center-tapped secondary?

t

ehold appliances. You can't use two taps of the three phase power for that unless using a particular type of delta transformer that center taps a coi l. But then it's still 240 volt split phase.

neutral/ground because that's what we use for a return. True three phase doesn't need a neutral for power purposes.

neutral and 208V from phase to phase

to phase so we can run our ovens an furnaces.

e 240 volt split phase handy. If we did use 208 volts everything would pro vide 25% less power.

commercial sites where they have 3 phase and they just give 208 volts to t he connection rather than bother with 240 resulting in slower charge times.

nient with out a neutral, having to run everything on 400V

a distribution that you can pick lines off of for low voltage in the home. We have three phase on the power distribution, it's not very often brought down the street and hardly ever brought to a home. Pretty much commercial only.

yes,

It's just a dumb meter that independently measures the three CT inputs via A/D, which looks lke a 3:1 mux input. And quoting their own datasheet, phas ing of the CT isn't even required.

"Current Transformer Phasing: Since the ACM3P only measures amperes, transformer phasing is not required. ?Phasing? ref ers to the direction of the current-carrying wire as it passes through the primary hole of each of the three built-in current transformers."

The two phase jazz just means the meter is set up to display L1 and L2 chan nels, versus L1, L2 and L3 in 3-phase mode.

Thanks, Fred, it makes sense now.

--
 Thanks, 
    - Win

The term split phase is not familiar to me. Thank you.

I am with you on that one, and even recall seeing an advertisement for zener protected CTs a couple decades ago while working with protective relaying. I don't know why it didn't catch on, but perhaps from lack of demand from utilities, which have been using unprotected CTs for over 100 years. I just pulled out my copy of the first edition of "Principles of Alternating Current Machinery" by Ralph Lawrence, published in 1916, and it has 3-1/2 pages on current transformers. Included is a warning about opening the secondary under load as this will cause core magnetization and erroneous low output until the core is demagnetized. I once saw a $2k CT replaced because no one knew to demagnetize it. Lawrence does not mention the problem of insulation breakdown, and I think that this was prevented in the scrapped $2k CT by a small arc gap built into the integral shorting bar assembly, which is closed to service attached wiring and instruments without disrupting power - a fairly common feature in large CTs,

With large CT secondaries normally rated at 25 VA 5A they are not out of spec below 5V RMS, 1v for small 5 VA CTs, with some overload/surge capacity required, so I think back to back zeners are required. (The internal burden approach does not work for using one CT with multiple series wired meters and protective relays as is normal utility practice.)

There is nothing to stop us from bolting those missing zeners to the terminals of our expensive CTs however.

Glen

se

it

usehold appliances. You can't use two taps of the three phase power for th at unless using a particular type of delta transformer that center taps a c oil. But then it's still 240 volt split phase.

om neutral/ground because that's what we use for a return. True three phas e doesn't need a neutral for power purposes.

o neutral and 208V from phase to phase

se to phase so we can run our ovens an furnaces.

ave 240 volt split phase handy. If we did use 208 volts everything would p rovide 25% less power.

at commercial sites where they have 3 phase and they just give 208 volts to the connection rather than bother with 240 resulting in slower charge time s.

venient with out a neutral, having to run everything on 400V

s a distribution that you can pick lines off of for low voltage in the home . We have three phase on the power distribution, it's not very often broug ht down the street and hardly ever brought to a home. Pretty much commerci al only.

It's not familiar to most people in the US either. They want to call it tw o phase.

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  Rick C. 

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Why do you not use standard terminology: DELTA and WYE?

Too bad. AFAIK, there is no two phase power commercially supplied in the US.

John S wrote in news:r7s3b0$ukd$ snipped-for-privacy@dont-email.me:

We get our 240 VAC from a pole transformer (or ground) off ONE phase of a three pase HV distribution system. From that ONE phase, there is a center tapped transformer which is 180 degrees out of phase with each other, so each could be referred to as a phase, but only within the US residential feed setting. So we get the full single phase 240 volt feed from the pole, but we ALSO get the center tap from that secondary, so that within the house, we can distribute

120 VAC branches, which are 'safer' from a human shock death risk potential view. We feed the full 240 V to large consumption items like electric dryers and our kitchen range.

So what gets 'supplied' is a center tapped supply of a tranformer secondary of 240 VAC, which is essentially two 120 VAC feeds into the home. One of the main features of this system is faulty return handling, and at the pole, lightning strike management, as a hit on an ungrounded floating feed could end up sraying lightning righ inside your now ablaze home.

snipped-for-privacy@decadence.org wrote in news:r7s44j$1lps$ snipped-for-privacy@gioia.aioe.org:

fault return, not 'faulty' spraying not sraying

We're supposed to have the center-tap neutral wire solidly connected to a water-pipe ground or similar. I've observed that for many older installations, the two sides of the 240V come in as two insulated wires, but the center tap is connected to a steel cable supporting the pole-to-house wiring. This saves adding a third electrical wire. But the cable-clamp connection is a weak spot, that can become corroded and fail.

--
 Thanks, 
    - Win