It's understandable that a CT, needs to be shorted (or a burden resistor) if there's nothing connected to its output, otherwise it would work as a step-up transformer.

But why is it that energy meters (example) show in their datasheets(p.9, fig1-8) that the CT's should be connected to earth?

As they state in the user manual:

Current measuring input terminals must be connected through a R/C Measuring transformer with one lead connected to Earth. Direct connection to the line voltage is not allowed.

I wonder what is the technical/scientific reason behind this.

  • \$\begingroup\$ That's not the earth ground symbol. I think they're just showing that the three current transformers have one end if their windings connected together, and that it is connected to the local ground (which is presumably shared with the readout electronics). \$\endgroup\$ – Connor Wolf Jan 26 '12 at 10:43
  • \$\begingroup\$ But the quote in the question is taken from their manual, where they say one lead of each CT must be connected to Earth. Also on figure 8, for instance, theres a single CT and still they seem to connect both leads to the meter and while one continues to ground. Also this connection scheme isn't particular to this meter/brand, its common practice. \$\endgroup\$ – Bruno Flávio Jan 26 '12 at 11:01
  • \$\begingroup\$ In that case, it's almost definitely a safety precaution then. Any failure in the CT will lead to an immediate tripping of any upstream GFCI, or a blown circuit-breaker. \$\endgroup\$ – Connor Wolf Jan 26 '12 at 11:22

100% sure answer may need much digging but

It is almost certainly for safety reasons.

A current transformer output produces its result in splendoured isolation - it floats relative to the universe (or ground or line or ...). Stray leakage and capacitive coupling MAY cause it to float to undesirable voltages so referencing it to something is a good idea [tm].

Connection to phase would achieve this - it will still function the same BUT this may make your eyes water or worse if the insulation is not perfect or you find some exposed part of the circuit to touch.

SO connecting it to ground at some point in its circuit may make your days longer on the face of the land - or those of your client or family or ...

  • 1
    \$\begingroup\$ @russelmcmahon: So, if I understand correctly, in the theory realm if 3 CT's are individually connected to 3 analogue ammeters the readings would be accurate, as each CT-ammeter pair would be in splendoured isolation (still a safety hazard as the potencial difference to the ground is unknown). The problem arises when using digital equipments where the 3 CTs will be connected to the same equipment, where their individual voltages float will affect readings, requiring them to be connected together to some reference. Of course, the safer reference to connect them into is the ground. \$\endgroup\$ – Bruno Flávio Jan 26 '12 at 11:32
  • \$\begingroup\$ @Qsp - Yes. Nicely put. That's how I understand it. \$\endgroup\$ – Russell McMahon Jan 26 '12 at 11:59

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