Current Transformers (CT) are devices that function as an interfacing solution between instrumentation devices and high currents. They can reduce or multiply an AC (alternating current), so if you have large, primary currents that you need to scale to a smaller current that’s easier to measure, you can do it with a Current Transformer. What determines the relation between the input and output currents is the ratio between the primary and secondary windings.

Reducing the Current Burden

Current Transformer standards differ between the European and the North American standards in relation to the secondary current: – 1 Ampere secondary CTs are used mostly in Europe (and other countries that follow their standards), while in North America, 5 Ampere secondary CTs are very common.

The old type of protective relays and electromagnetic meters required a heavy load or high “burden” to operate. To have the desired effect in the relay or meter, they required a relatively high power, so these CTs were designed to reduce the current and also maintain the burden required to operate the protective relay. As technology changed from electro-mechanical to electronic instruments the burden got reduced drastically, modern instruments (meters) can use smaller burden CTs because they don’t require high power to operate.

More Accurate and Smaller MilliAmp CTs

CTs with a 100mA secondary output (or MilliAmp CTs) can be just as accurate as larger 5 ampere secondary CTs and are inherently smaller. Their size also makes them ideal for retrofit use. When ensuring the safety of electrical installations, electrical inspectors are concerned with the fill space that CTs would occupy inside the distribution panels. MilliAmp CTs can easily fit in any distribution panel of an older building that’s ready for retrofitting. According to Section 373-8 of the National Electric Code (NEC):

• Splices and taps inside the panelboard shall not fill the wiring space at any cross-section to more than 75%.
• Conductors shall not fill the wiring space at any cross-section to more than 40%.

CT Temperature and Safety

Current transformers take the primary current to produce a current on the secondary side to match the burden connected to it (on the secondary side). In theory, an open circuit is an infinite resistance which would generate an infinite voltage (and 5-ampere secondary CTs with no burden can induce very high voltages). The only resistance is that of the copper conductor wire used in the secondary winding. Another way to explain it is simply because there is only one turn in the primary and multiple turns in the secondary winding, the Current Transformer behaves as a step-up transformer, one that raises the voltage. When the standard 5 Amperes CT secondaries are installed, shorting blocks are always required to prevent high voltages present on the secondary terminals. If the appropriate precautions are not taken to prevent burdening (open circuiting) the output, the induced voltages may cause severe injuries or even fatal accidents.

Self Shorting MilliAmp CTs are also available in the market. They close the secondary circuit with a small burden resistor and a diode to ensure the safety of the installer.

When used with accurate solid-state electronic meters, MilliAmp CTs are an ideal companion. The benefits and advantages of MilliAmp CTs are the smaller cost, smaller size, and safer conditions in open circuit conditions. Thanks to built-in voltage suppression devices, mA CTs eliminate the potential danger of electrical shock and fire.