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CT sensors - An introduction

Yhdc Current Transformer: SCT-013-000

Basics

Current transformers (CTs) are sensors that are used for measuring alternating current. They are particularly useful for measuring whole building electricity consumption (or generation for that matter).

The split core type such as the CT in the picture above, is particularly suitable for DIY use it can be clipped straight on to either the live or neutral wire coming into the building without having to do any high voltage electrical work.

Like any other transformer, a current transformer has a primary winding, a magnetic core, and a secondary winding.

In the case of whole building monitoring the primary is the live or the neutral wire (not both) coming into the building itself and goes through the hole in the CT. The secondary winding comprises many turns of fine wire housed within the casing of the transformer.

The alternating current flowing in the primary produces a magnetic field in the core, which then induces a current in the secondary winding circuit [1].

The current in the secondary winding is proportional to the current flowing in the primary winding:

Isecondary = CTturnsRatio x Iprimary

CTturnsRatio = NoTurnsPrimary / NoTurnsSecondary

The number of secondary turns in the CT pictured above is 2000 and so the current in the secondary is one 2000th of the current in the primary.

Normally this ratio would be written in terms of currents e.g. 100:5 (for a 5A meter scaled 0 - 100A). The CT above would normally be written as 100 : 0.05.

Burden resistor

A current output CT needs to be used in conjunction with a burden resistor. A burden resistor completes (closes) the CT secondary circuit. The burden is sized to provide a useful voltage signal that is proportional to the secondary current. It also needs to be of low enough value to ensure the core does not saturate.

Isolation

The secondary circuit is galvanically isolated [2] from the primary circuit. (has no metallic contact)

Safety

In general a CT must never be open-circuited once installed, and is potentially dangerous if it is open-circuited.
 
If open-circuited with current flowing in the primary the transformer secondary will attempt to continue driving current into what is effectively an infinite impedance. This will produce a high and potentially dangerous voltage across the open secondary [1]
 
Some CT's have built-in protection. This can be either protective zener diodes in the case of the SCT-013-000 that is recommended for use in this project, or if the CT is a voltage-output type, it will have a built in burden resistor and so cannot be open-circuited.
 
Installing a CT
 
The primary winding of the CT is a wire that carries the current you want to measure. If you clip your CT around a two- or three-core cable that has wires carrying the same current but in opposite directions, then the magnetic fields created by the two wires are equal and opposite and will cancel. Your CT will have no output. [3]

References and further reading

Test report: Yhdc SCT-013-000 Current Transformer

[1] Wikipedia article on current transformers

Elktor Introduction to current transformers

[2] Wikipedia article on Galvanic isolation

[3] CT and AC power adaptor installation and calibration theory