Kilo watt hour rating

[hisham1986]

Hi guys,

i would like please to know what will happen in case a phase current out of the three phase currents passing through the kilo watt hour meter exceeds the rating of the meter,does the meter burn??

Thank you in advance.

 

[gar]

130403-1533 EDT

hisham1986:

Most physical devices do not have a very precise threshold point of failure, and many are time dependent.

Some different kinds of examples:

A steel tensile test sample will have a tensile break point that is quite predictable on a tensile test machine. Take the same type of sample and subject it to a lower stress level than the tensile test machine failure point, cycle a very large number of times, and you can produce a fatigue failure.

Aluminum and eutectic tin-lead solder have much poorer fatigue failure characteristics than does steel.

A Zener diode when reverse biased conducts very little current until a threshold point is reached. Just above the threshold value the Zener becomes a very low impedance and will conduct a large current if the current is not limited. If an excessive amount of current flows the Zerner burns out. Thus, a very small change in voltage can easily produce a failure.

An automotive starter motor can produce a large amount of output power for a short time, but would burn out quickly if the on time was not limited, and there was not sufficient cool down time between uses.

What is your spinning disk kilowatt-hour meter made of? Conductors, magnetic material, and rotating elements. Small overloads will produce an accuracy problem. Sufficiently large overloads, and/or extended overload time will cause some failures. This is not a sharp threshold type of problem like the Zener diode.

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[fmtjfw]

Depends on how much. A class 320 KWH meter is applied to a 400A service here. 320A is probably it's continuous load, but power company equipment usually is conservatively designed and can take overloads. In that sort of mechanical (or solid state) meter the current coils are probably just a hunk of bus bar.

 

[hisham1986]

So if we assume that the kilowatt hour meter is rated 20Amps three phases,and one phase is passing about 40 amps,this will cause it to fail right?failure is considered to be as a "burning" of the meter here?

 

[gar]

130304-1622 EDT

hisham1986:

If you are speaking of a spinning disk kWh meter, and not a wattmeter. Then internal to the meter I suspect you could run the 40 A (an overload equal to the rating, therefore 4x the power in the internal wiring) without burn out. If this was a 200 A meter and you ran 400 A thru it for a moderate time that might be a problem. It is mostly a problem of heat.

The external connections of the meter to the base may be a different problem. Low force at the contact points in a plug-in meter.

Do you have a problem or is this just to give you and understanding.

For example: A #12 copper wire in nominally rated at 20 A, and this rating is defined for insulation material and environment. The fusing current (burn out point) of this wire is much higher.

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[hisham1986]

gar:

yes indeed i am speaking of a spinning Disc KWH meter,rated at 20 Amps,and the current passing thru it is 40 amps,so as per your reply i guess it will burn...

 

[Hv&Lv]

gar:

yes indeed i am speaking of a spinning Disc KWH meter,rated at 20 Amps,and the current passing thru it is 40 amps,so as per your reply i guess it will burn...

How long is the 40 amps going through it?

Change the CT to a bigger size.

 

[hisham1986]

well its gonna be mostly more than 24 hours,and is it possible to change only the CT or should i replace the whole kit with a higher rating i.e: buy a total new 40 Amps KWH meter??

 

[gar]

130405-1332 EDT

hisham1986:

Whether your meter will burn out or not is a function of temperature rise and the materials used.

i do not know the criteria used in designing a kWh meter, but I will provide you with a rough reference.

Standard transformers are made with insulated copper wire. From long lost memory I pulled out a guess at 1000 circular mils per ampere for copper wire.

A transformer I have from a major manufacturer rated for continuous duty has secondary wire that is about #11 and rated load is about 11 amperes. #11 is 8234 circular mills. Or 749 square mills per ampere. My memory could be bad, but also better insulation is available today.

Thus, based on my transformer I would expect that a kWh meter for 50 A would use wire no smaller than 8234*50/11 = 27427 circular mills. This would imply #4 copper wire. What is the wire size in the current coils of your kWh meter?

Your 24 hour on time would be a continuous criteria. As a wild guess the current coils in the kWh meter would probably reach steady state temperature rise within an hour, maybe 10 minutes.

In a low current range spinning disk type of meter I doubt that current transformers are used unless it was for high voltage.

Overloading a kWh meter will probably introduce calibration errors.

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