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Replacement of galvanic Isolation by two Protective Earth wires to comply with leakage current demands under normal and single fault conditions

IP.com Disclosure Number: IPCOM000241789D
Publication Date: 2015-Jun-01
Document File: 4 page(s) / 103K

Publishing Venue

The IP.com Prior Art Database

Abstract

In many power grid fed applications galvanic isolation is used for safety reasons. It makes sure that leakage currents will stay below safe limits for any person touching the equipment in both normal operation conditions as well as in single fault conditions. Often the galvanic isolation is implemented with a 50/60 Hz transformer. This is, however, a bulky, heavy and costly component, especially in applications with high power levels (> 2 kW). The invention proposes the galvanic isolation to be exchanged with 2 protective earth wires from the wall socket to the equipment and a relay, while maintaining leakage current demands

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Replacement of galvanic Isolation by two Protective Earth wires to comply with leakage

current demands under normal and single fault conditions

Abstract of the invention:

In many power grid fed applications galvanic isolation is used for safety reasons. It makes sure that leakage currents will stay below safe limits for any person touching the equipment in both normal operation conditions as well as in single fault conditions. Often the galvanic isolation is implemented with a 50/60 Hz transformer. This is, however, a bulky, heavy and costly component, especially in applications with high power levels (> 2 kW).

The invention proposes the galvanic isolation to be exchanged with 2 protective earth wires from the wall socket to the equipment and a relay, while maintaining leakage current demands.

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Detailed description of the invention

Background of the invention

If an electrical application is fed from a 50/60 Hz power grid, leakage currents will be generated through all capacitors that connect between the power lines (Line and Neutral) and the protective earth (figure 1). Many of these capacitors are of a parasitic nature (C4), but a number of them can be deliberately placed, such as the Y-capacitors in common mode power entry filters (C2 and C3).

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Figure 1: Capacitances within an application connected to a power grid.

As long as the protective earth is connected properly, these leakage currents will flow through the same and will not pose a risk of electric shock to any person touching intentionally or unintentionally the equipment. However, if the protective earth has a failure and is not connected anymore to the protective earth of the rest of the installation, these leakage currents will start to flow through any person that touches a galvanically conducting part on the outside of the equipment, see figure 2.

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Figure 2: Leakage current flowing through a person touching the application if the PE has a failure.

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To prevent that these leakage currents form a health or life risk for anyone that gets into contact with the equipment under these conditions, the leakage currents must remain below certain levels. These levels depend on the application, e.g. for medical equipment maximum leakage current levels are given in the IEC 60601 standard.

The most common way to prevent the above mentioned situation is to implement galvanic isolation by means of a transformer, as depicted in figure 3. The leakage current in this situation is dominated by the parasitic capacitance of the transformer (C5), as this capacitance is in series with all other capacitances in the system. As long as the transformer capacitance is small enough, the maximum leakage current, both under normal and single fault conditions, is always guaranteed.

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Figure 3: Galvanic isolation (L3, L4) to limit the maximum leakage current

Problems or disadvantages overcome by the invention

Although a galvanic isolation works fi...