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Hello,
When looking at the ET112 wiring diagram it shows two ways of which this can be wired. Can the ET112 have the main neutral wired through it like figure 3? My electrician wants to check to make sure these are designed to have the main neutral fed through.
This is the UK and we have a Single phase, 100amp supply fed via 25mm tails after the meter.
Many thanks
Ours is wired as per fig 1. Din rail mounted fuse holder.
My electrician seems to think its really bizzare to fuse the neutral.
It is counter intuitive but the reason is sound.
The meter is capable of measuring 100A in the line conductor, and only measures current in the line conductor. The neutral is only required to provide power for the operation of the meter. The operating electronics are very low current and made up of very small conductors. If something went wrong inside the meter, the only current limiting device would be the one upstream of the meter and this could be 100A, it could be that the DNO cutout is the first upstream current limiting device. The meter electronics would explode and burst into flames and the upstream circuit protection might not even be activated.
So the solution is to fuse the neutral conductor with a much more appropriately sized protection device. If a fault develops inside the meter the small fuse will open and the meter's electronics will be disconnected. Sure, it will still be live, connected to the line conductor and probably still happily passing the current in the line conductor, but it would not be on fire.
The meter remaining live after a fault is not a problem as it is not a final circuit and should be housed in and appropriate cabinet (consumer unit) where other devices also remain live after performing their circuit protective functions.
To be honest, I thought it was really bizarre and not sure what type of fuse you'd use for this either. Mine uses a 1.5mm² unfused wire to the common neutral in the consumer unit. As @WKirby mentioned, it's only protecting the circuitry with the meter.
What is the equivalent current rating of the fuse required to protect the ET112?
I'm beginning to think in the UK a 1A 230V cartridge fuse will - the kind you find inside a typical low current mains appliance. Or better yet a Type A MCB with a rating of 1A to 2A on the neutral leg of the ET112.
Which would be best?
Something like this is what I have used with the specified F315A fuse. https://uk.rs-online.com/web/p/fuse-holders/2375259?cm_mmc=UK-PLA-DS3A-_-google-_-CSS_PLA_UK_EN_Fuses_%26_Circuit_Breakers_Whoop-_-Fuse+Holders_Whoop-_-2375259&matchtype=&aud-842313741538:pla-305324206926&cq_src=google_ads&cq_cmp=16178262939&cq_term=&cq_plac=&cq_net=g&cq_plt=gp&gclid=EAIaIQobChMI1aGvmITU-wIVkbrtCh3R2ABAEAQYASABEgKRhfD_BwE&gclsrc=aw.ds
Dupe...
We have done it both ways depending on the layout of the tails. Few things to note, L and N tails must enter or leave a metal enclosure through the same hole which complicates things a little on some enclosures. We generally use a plastic one for this so N goes in and out the bottom, L in and out the top. Using a 32mm tails gland means the earth hole can be used for the data cable.
Dupe...
