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Why is the accuracy of multifunction testers so low

Alan B

I am working on a scenario at the moment where my customer has an EV charger cutting out due to low and also possibly high voltage.
My KT65DL is indicating voltage as low as 214V and seeing swigs from 214 to 246 in a 1 hour period. The DNO are saying they don't trust the values being given and are declining to put a voltage logger in, asking for data from the EVCP manufacturer, which is unlikely to be available as it's Tesla and probably no more accurate.

But it's accuracy is +/- 2% plus 4 digits, so effectively +/-5V, which means it has very little value for checking the supply voltage is in range, also doesn't help that the sampling is quite slow and I assume minimums are being missed.

To my knowledge it's not that expensive to build reasonably accurate voltage meters, maybe AC meters cost more? But given multifunction testers cost £500 or more why is the accuracy so low?

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  • 0

    A Fluke blog, including a video, remember what you are trying to measure is a moving target and it’s an event that needs to last for more than four seconds, so there could be a lot of near misses lasting several seconds.

    www.fluke.com/.../what-is-true-rms

  • 0 in reply to Sparkingchip

    And then there's the further complication in this case - I've only just thought about this - that it all depends what the cut-out circuit on the EV charger is working on, which almost certainly won't be true rms!

    So actually the only way to know whether it's the voltage swing that's causing the charger to cut out is to have a meter that measures the voltage in the same way (and at the same speed) as the voltage monitoring circuit in the EV charger...which I can well imagine is just a tiny transformer, a diode, and a capacitor...

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  • 0 in reply to Sparkingchip

    And then there's the further complication in this case - I've only just thought about this - that it all depends what the cut-out circuit on the EV charger is working on, which almost certainly won't be true rms!

    So actually the only way to know whether it's the voltage swing that's causing the charger to cut out is to have a meter that measures the voltage in the same way (and at the same speed) as the voltage monitoring circuit in the EV charger...which I can well imagine is just a tiny transformer, a diode, and a capacitor...

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  • 0 in reply to Andy Millar

    Yes, both the Open PEN device and tester need to be testing in the same way to get the same results, BS7671 says the measurements should be RMS, but a lot of EVSE has been developed since the relevant regulations were written and implemented, so can comply if they vary from what is written in the book, because there’s an allowance for new technology.

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