I'm looking for a reasonably simple way to measure consumption of one circuit on a consumer unit. I installed the CU myself so I'm not afraid of doing a bit of poking around.
It's a circuit on a 20 amp MCB. There are two spare MCBs in the CU. Can one get DIN rail 'meters' that will measure total kWh. The two spare MCB slots are side by side so a double width space is available.
Yes, analogue or digital versions available, e.g.
Only needs a single width module for single phase
Yes; if you are recharging for electricity you will need a MID-approved one
... and a quick search shows me there are such things! E.g. :-
Depends a bit on how "elegant" the solution needs to be really...
A rough and ready option is using a normal clamp meter - clip it round the live conductors on the top of the MCB and you will get a direct reading.
If you want a slightly more "closed" but rough and ready option, then run a loop of T&E out of the CU and back in. Using just the live conductor wago/crimp the circuit wires to the T&E, and connect the other end to the MCB. You now have a place to stick a clamp meter externally and take a reading, but the whole thing is now "safe" with no accessible live parts and the external measurement loop cable is double insulated.
The next option might be a variation on some of the power monitor projects you can find here:
I've just looked at a different make and it seems to have two neutral connections at one end and a live in and live out at the other. I presume that it does not connect to the busbar, you connect your load-side live and neutral to top and bottom, add a link from the other neutral to the neutral rail, where your load neutral was previously connected and add a link from the MCBs live output to the meter's live input.
I have a DDM18SD (ebay, Amazon, Aliexpress) - it's Chinese but seems to work ok. A 3kW immersion heater came out at 2997W which was pretty good. It does Modbus and pulse output for about £20. Probably not MID approved.
The main thing is it flicks between various displays - voltage, current, frequency, power factor, power, energy consumption. That is handy for testing (and for Modbus recording) but might make it more annoying to read as you have to wait for the kWh figure to go past.
(It may be this behaviour can be changed via Modbus - I haven't tried that)
Theo
On the above (a DDS353) the live in and live out are on the top, and neutral at the bottom. On my DDM18SD it's live in at the top, live out at the bottom, neutral in (and pulse/modbus) via a second level terminal. Either way would suit, as long as you have a busbar-free space at the bottom.
However you might want to consider whether the meter is before the MCB or after it. After the MCB means the MCB protects in case of a fault in the meter, but before means the meter stays powered when the MCB is turned off. In my case I go MCB -> meter -> contactor, so the load is controlled by the contactor but the meter stays powered if the load is off (which it will be, most of the time).
Theo
Clip on ammeter or watt meter would be my first choice - assuming you can find somewhere with that circuit's live or neutral on its own.
How is it wired though? Does it need connection(s) where the busbar is on a CU? From the pictures I can find it looks as if it does have its connections where the bus-bar normally goes so would be difficult for me to install.
But I don't have a busbar-free space, that's the whole point. The MCB whose power I want to monitor is in the middle of a 20-way CU with no spaces at either end. The spare spaces are in the middle.
Does it lose its readings if it's powered off then?
I don't see how this will work then. Any DIN-rail device is likely to have contacts where the busbar goes - that's the standard high-current location. The extra wiring for neutrals etc may go to a second 'top deck' of contacts, but typically the live goes through the lower deck.
You want a chain: busbar-live -> MCB -> meter -> load
but busbar-live -> MCB means the live emerges at the top of rail. If the meter took in live at the top position of the lower deck (like the DDM18SD) the output would be where the busbar finger goes. If the meter takes in live at the bottom position on the lower deck then it'll be wired directly into the busbar and will miss the MCB.
Either way the busbar is going to block doing what you want. The only recourse is to find a device where the lower deck bottom position isn't used, but that seems unlikely.
No, but it's an electronic display so you can't take readings if it has no power as the display is blank. Since my contactor is remotely controlled, it is awkward to power the load just to take readings.
Theo
Can you not unclip each MCB to one side of the spares, in turn, and move them over two spaces, so that you end up with two spaces at the end?
I would expect that they either have internal power backup or (more likely) write to EEPROM to retain information on power loss - indeed they cannot lose the information or they'd not be MID certified - and it is unlikely that non-certified ones lose readings either.
Possibly, but it does rather spoil the nice, consistent set-up of the CU. Maybe I should simply get a small (2,3,4 position) CU and install the meter there. That wouldn't be too difficult and would leave the main CU undisturbed.
Yes, OK, that would appear to be the case so it's not an issue.
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