Can you place CT around all cables rather than just live? (UK)

[Cavester]

Hi,

As your probably aware UK consumer units are tight.

In UK the general makeup of an electrical (twin and earth) cable is 1 X positive, 1 X neutral and 1 X earth which is contained inside an outer sheath.  Once inside of the consumer unit the 3 wires are exposed and screwed into their relevant connections, with the positive going into the RCBO (trip switch).

I note by the instructions the CT clamps need to go round the positive wire going into the RCBO, however my question is, can the clamps actually go around the whole twin and earth cable outside of the unit instead?  Will this still work or has it to go only around the “live” wire inside of the consumer unit to be able to work?

Same for the incoming from the grid, which in my case is armoured cable.

 

Thanks in advance.

[John Polasek]

Unfortunately, no.  The CT works by measuring the total sum of current passing through the core;  since (unless you have a short to ground at the load, a dangerous electrocution hazard that trips Ground Fault Interrupter devices) all the current that goes OUT through the positive wire comes back IN THE OPPOSITE DIRECTION through either the neutral or ground wire, the sum will be 0 if all three are inside the core.  That is why the unit is usually placed inside the circuit box where the “positive” (hot, color coded black or red in the US) wire is separated to go to the circuit breaker, while the neutral (common, color coded white in the US) goes to a common bar and the “earth” (Ground, color coded green or bare wire in the US) tied to a common bar that itself is tied to a stake driven into the ground.

[djwakelee]

The CT must go on the hot (live) power feed wire only.  If you were to clamp it across all conductors like you suggest (live / neutral / ground), the live and neutral current flow would cancel out (subtract), and you would read zero current flow under all conditions.  This is not an Emporia thing specifically – true for monitoring any current with a CT in general.

One thing you can do in the UK with single phase is that you can reverse the CT, and clamp over just neutral instead of live.  That will also work (since AC current flows between live and neutral and vice-versa), but I’m not sure if that helps your situation or not.

• This reply was modified 2 years, 2 months ago by djwakelee.
• This reply was modified 2 years, 2 months ago by djwakelee.

[Cavester]

Thanks for the response, I appreciate it.
<p style=”text-align: left;”>Tbh I think I’ll get the clamps on the live inside the CU, I was just wondering if there was an easier way.  Now I understand how the CT’s work and what you have described I get it.</p>
Thanks, I’ll install the CT’s in the CU and Emporia init external.  Note to Emporia, it’s worth shielding the Emporia control unit power connector so we can do this according to regs in UK.

Also, please bring out a UK 230v energy monitor plug, pretty please 🙂

[Author]

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