# Negative reading while measuring Neutral

Hi! I have an 100A CT sensor on my neutral line. With normal usage the wattage sits at negative. I must have put the clamp the wrong way around. But occasionally when I use some appliances, it goes positive. Which I don’t quite understand.

Are there any reasons why my neutral would go “negative” ?

Welcome, Daniel, to the OEM forum.

What sort of electricity supply do you have there in Finland? The answer could lie in that.

My understanding is you probably have a three-phase supply, would that be the case?

And would it be a genuine 4-wire supply (three phases and neutral, not counting a protective earth)?

If it is, are you are using more than one phase to supply your house?

Hi Robert.

I have three phases coming in to my apartment.

I’m using two phases from my understanding to power my apartment both behind individual RCD’s.

Last phase only powers the fire alarm and a light.

Oh dear. A quick explanation of three-phase supplies is needed: Ideally, the load drawn on each phase will be equal. The timing of the three voltage waves means that at any instant, the current flowing in on one phase is exactly balanced by the current flowing out on the other two phases, with the result that there’s no current at all in the neutral wire - and in fact, if you have a big load like a large industrial motor, it won’t have a neutral wire at all. Unfortunately, in the real world, you rarely get an exact balance, so the neutral carries the imbalance current, actually it’s the vector sum of the currents in the three phases returning to the supply.

There’s an explanation of three-phase supplies here: Learn→Electricity Monitoring→AC Power Theory→3-Phase Power→3-Phase Power

So, what you’re actually measuring is the imbalance current, and that doesn’t relate to your total usage except in one important case: when you’re only using one phase - the one that the a.c. adapter is connected to.

Now the reason for my “Oh dear” is the emonPi was designed for use only on a single phase (or split phase) supply, it can’t be modified for three-phase use. What you needed was an emonTx with the emonBase. You can add an emonTx, and download the three-phase sketch for it, which will allow you to measure the total load of your house. You’d retain your emonPi to run emonCMS on, and you’d have two current inputs that you could use, but they’d both need to be on the same phase as its a.c. adapter.

I’m sorry about this, if you’d asked what we advise before purchase (it does say ‘single phase’ in the description) I could have steered you in the right direction. Run what you intend to do past one of us before you buy anything.

If your emonPi is in “as new” condition, it might just be worth asking if the shop will take it back - but no promises.

Ah! well that explains it…

Well what a shame! I guess I’ll have to grab an EmonTX then at some point.

Thanks for your help anyway. Appreciate it.