Transformer Core for Ground Fault Circuit Interrupter (GFCI) (ELCB Earth leakage circuit breaker)

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1 Transformer Core for Ground Fault Circuit Interrupter (GFCI) (ELCB Earth leakage circuit breaker) Functions of GFCI: GFCIs are devices that interrupt power in your home in case a person gets an electric shock. They are fast-acting devices that, in many cases, resemble normal circuit breakers. They can trip the circuit at fault current as low as 5mA in few milliseconds. The GFCI operates by sensing the difference between the currents in the Hot and Neutral conductors. Under normal conditions, these should be equal. However, if someone touches the Hot and a Ground such as a plumbing fixture or they are standing in water, these currents will not be equal as the path is to Ground - a ground fault - and not to the Neutral. This might occur if a short circuit developed inside an ungrounded appliance or if someone was working on a live circuit and accidentally touched a live wire. The GFCI differential transformers are normally wound with 1000 turns of 38 AWG to 40 AWG magnet wire and are placed in pinned core holders for easy circuit board mounting. The pins can be bent for horizontal or vertical mounting depending on the construction of the GFCI device. How does a GFCI work: L live conductor N neutral conductor Electromagn et controlled by electronics Secondary

2 winding of Current transformer. Transformer core Test switch GFCIs typically test for the following condition: A Hot to Ground (safety/ea rth) fault. Current flows from the Hot wire to Ground bypassing the Neutral. This is the test that is most critical for safety. A Grounded Neutral fault. Due to miswiring or a short circuit, the N and G wires are connected by a low resistance path downstream of the GFCI. In this case, the GFCI will trip as soon as power is applied even if nothing is connected to its protected (load) circuit. To detect a Hot to Ground fault, both current carrying wires pass through the core of a sense coil (transformer). When the currents are equal and

3 opposite, there is no output from its multiturn sense voltage winding. When an imbalance occurs, an output signal is produced. When this exceeds a threshold, a circuit breaker inside the GFCI is tripped. To detect a Neutral to Ground fault there is a second transformer placed upstream of the H-G sense transformer. A small drive signal is injected via the 200 T winding which induces equal voltages on the H and N wires passing through its core. Most GFCI devices work in the same way; a sensing transformer compares the power going through the live wire to the power being returned through the neutral wire. If a difference is present, power is going to ground in an unintended manner and causes an imbalance in the transformer. As a result, the transformer conducts power which is amplified and applied to a coil which trips the circuit breaker. GFCIs and safety ground: Despite the fact that a Ground Fault Circuit Interrupter (GFCI) may be installed in a 2 wire circuit, the GFCI does not create a safety ground. In fact, shorting between the Hot and Ground holes in the GFCI outlet will do absolutely nothing if the GFCI is not connected to a grounded circuit (at least for the typical GFCI made by Leviton sold at hardware stores and home centers). It will trip only if a fault occurs such that current flows to a true ground. If the original circuit did not have a safety ground, the third hole is not connected. What this means is that an appliance with a 3 prong plug can develop a short between Hot and the (supposedly) grounded case but the GFCI will not trip until someone touches the case and an earth ground (e.g., water pipe, ground from some other circuit, etc.) at the same time.

4 What HIPERM can offer? Current Transformer (CT): This is made from 80% NiFe material laminations which are stacked together in a casing. Copper winding is provided as per the requirement of the application. NiFe Soft Magnetic Alloys and their Properties Sr Properties 80% NiFe Mumetal 48% 36% NiFe 1 Permeability in DC Bs(1) Tesla Bs(2) Tesla Hc(2) (A/m) µ4 (3) µmax Permeability in AC µ4z

5 2 3 In Cold worked condition In Annealed condition µmax Chemical Composition % Ni % Mo Nil Ni % Si % Mn % C % Fe Balance Balance Balance Bala Physical Properties Density 8.7 g.cm² 8.7 g.cm² specific Heat 460J.Kg¹. C¹ 460J.Kg¹. C¹ Thermal conductivity Mean CTE between C and +200 C Electrical resistivity Curie Point Coefficient of 1,-3 1, x magnetostriction at Saturation l / l Melting temperature C Saturation Induction Tesla 1.6 Hardness (HV) UTS ( Mpa) % PS (Mpa) Elongation (%) Hardness (HV) UTS ( Mpa) % PS (Mpa) Elongation (%) Grain Size

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