Editing Current transformer (section)

== Use ==
[[Image:Clampmeter.jpg|thumb|120px|right|Many digital [[clamp meter]]s utilize a current transformer for measuring [[alternating current]] (AC).]]
Current transformers are used extensively for measuring current and monitoring the operation of the [[power grid]]. Along with voltage leads, revenue-grade CTs drive the electrical utility's [[watt-hour meter]] on many larger commercial and industrial supplies.

High-voltage current transformers are mounted on porcelain or polymer insulators to isolate them from ground. Some CT configurations slip around the bushing of a high-voltage transformer or [[circuit breaker]], which automatically centers the conductor inside the CT window.

Current transformers can be mounted on the low voltage or high voltage leads of a power transformer. Sometimes a section of a bus bar can be removed to replace a current transformer.

Often, multiple CTs are installed as a "stack" for various uses. For example, protection devices and revenue metering may use separate CTs to provide isolation between metering and protection circuits and allows current transformers with different characteristics (accuracy, overload performance) to be used for the devices.

In the United States, the [[National Electrical Code]] (NEC) requires [[Residual-current device|residual current devices]] in commercial and residential electrical systems to protect outlets installed in "wet" locations such as kitchens and bathrooms, as well as weatherproof outlets installed outdoors. Such devices, most commonly '''ground fault circuit interrupters''' (GFCIs), typically run both the 120-volt energized conductor and the neutral return conductor through a current transformer, with the secondary coil connected to a trip device.

Under normal conditions, the current in the two circuit wires will be equal and flow in opposite directions, resulting in zero net current through the CT and no current in the secondary coil. If the supply current is redirected downstream into the third (ground) circuit conductor (e.g., if the grounded metallic case of a power tool contacts a 120-volt conductor), or into earth ground (e.g., if a person contacts a 120-volt conductor), the neutral return current will be less than the supply current, resulting in a positive net current flow through the CT. This net current flow will induce current in the secondary coil, which will cause the trip device to operate and de-energize the circuit - typically within 0.2 seconds.<ref>{{Cite web |title=Testing of electrical leads and residual current devices (RCDs) |url=https://www.health.wa.gov.au/Articles/S_T/Testing-of-electrical-leads-and-residual-current-devices-RCDs |access-date=7 June 2024 |website=Government of Western Australia Department of Health}}</ref>

The burden (load) impedance should not exceed the specified maximum value to avoid the secondary voltage exceeding the limits for the current transformer. The primary current rating of a current transformer should not be exceeded, or the core may enter its non-linear region and ultimately [[Saturation (magnetic)|saturate]]. This would occur near the end of the first half of each half (positive and negative) of the AC sine wave in the primary and compromise accuracy.<ref name=SHEE11/>