AC line current and voltage measurements for power calculations

i was wondering how companies who build Energy meters manage to make AC current measurements using shunt resistors and AC voltage measurements using voltage divider circuits safe to use in-spite of the circuit being exposed to direct AC line suuply without any form of isolation.. or do they use isolation? ive gone through a few schematics and i found that they mostly dont go for isolation methods probably for cost saving.

i wish to know how i am to obtain sine waves for both voltage and small value currents (mA range) as my load hardly draws any current. I understand i need to sample the sinusoidal waveforms later and obtain the values for current, voltage and powerfactor. My only concern is obtaining the waveforms safely.

can anyone who has worked with energy meters give me a good idea as to how i am to approach this situation.


thank you!

-pika :grin:

Hi,

It is not unsafe to measure mains voltages and currents without isolation.
If you all do this in an appropriate plastic box.

The question is how you interface the measured data.
* If it's a display behind an isolating, transparant plastic ... everything is safe.
* if you use bluetooth inside the plastic box to feed the data to a PC .... everything is safe.
* you use IrDA ... it's safe
* if you use plastic fiber or glass fiber .... it's safe

The problem arises when you want to use copper wires to transmit the data.

What interface do you want to use?

Klaus

Safety isn't a problem. Remember that absolute voltage alone is not dangerous, it is potential difference between two points that creates the safety issues. In line energy meters, the whole monitoring circuit is at (or close to) line voltage so as long as nothing at a different potential comes in contact there is no risk.

As Klaus points out, you need to be able to read the result somehow, and that requires interaction outside the circuit. For meters with digital readout it can obviously be read through a transparent window, for automated readings it is done using short range wireless, usually Bluetooth, Zigbee or something similar.

Brian.