Current Transformers (CT's) wired in series for two meters or relays
Connecting a protection relay and a kWh meter to the same set of three-phase CTs is possible as per typical schematic below (click to zoom in.)

Note polarity convention in use is "P2 towards protected object, S2 earthed" - your company's standards may differ.



CT classes for combined protection and metering

If using a CT for both protection and metering purposes, the CT must be rated for both. Most CT manufacturers should be able to accommodate this request.

In Australia, where we specify CT's to IEC / AS 60044.1, typical CT classes would be 10P20 2.5VA for protection and class 0.5M 2.5VA for metering. So the combined CT class would be "10P20 &amp; 0.5M 2.5 VA".

You can still connect a kWh meter to a protection-class CT (with no metering class), however the accuracy of the kWh meter will not be guaranteed.

Do not connect protection relays to metering CTs, as metering CTs are intentionally designed to saturate during fault conditions - not what you want on a protection relay circuit which is trying to measure those fault currents!

Possible damage to kWh meters during fault conditions

Metering equipment connected to a protection CT must be capable of withstanding the current that flows during a fault on the primary circuit. For example, if CT ratio is 800/5A and the prospective fault level is 25 kA, the secondary current may be up to 25,000 / [800/5A] = 156 A in the secondary circuit.

This fault may take up to 2 seconds to clear, depending on upstream protection settings. A typical kWh meter is only capable of withstanding 400 A for 0.5 cycles (10 ms) - equivalent to 28 A for 2 seconds. A secondary current of 156 A for 2 seconds will cause permanent damage to the kWh meter.

A typical solution is to provide a metering class, 1/1A ratio "interposing CT" in the secondary circuit, which will limit the maximum current in the kWh meter circuit. Interposing CT's are an off-the-shelf product available from all the usual vendors.

Further note that a class 10P20 2.5 VA CT will saturate at nominally 20x rated current only if the CT is loaded to the nominal burden, i.e. 2.5VA. Typical burdens for modern digital-type protection relays and meters are only 0.2 VA. So the current flowing in the secondary circuit can be much higher than you would expect from the CT specifications. If in doubt, use an interposing CT.