Electrical Engineering

What is the significance of a negative indication on a watt meter?

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November 30, 2011 2:21PM

Power meters are used to measure power used or produced by a customer or generator. A negative power meter reading at a costumer site would mean the costumer is generating electricity that is being pushed back out on the grid, or (more likely) the customer figured out how to reverse the meter! On older meters, reversing would cause the meter to count down so you could hide how much electricity you were using. Newer customer meters do not have this flaw - they add a magnitude of use (ignore the sign, or direction of power flow).

At a generating site, it would mean power is being sucked into the generator, instead of created by the generator.

Answer

I assume that you are referring to a wattmeter, and not an energy meter?

If this is the case, then a reversed reading simply indicates that either its voltage coil or its current coil has been connected the wrong way around. It can be rectified by reversing the connection to either coil (not both!).

When connecting a wattmeter, it's essential to observe the instrument terminal's 'polarity marks' when connecting the voltage and current coils to the circuit. This is because the currents passing through each of these coils must act in the same direction relative to each other or the instrument will read 'down scale' (i.e. backwards!).