Transformer Isolation

An in-depth look at transformer isolation. Electrical isolation is necessary to protect circuits, equipment, and people from shocks and short circuits, as well as to make accurate measurements. Isolation transformers are one way to go.

Introduction
Electrical isolation is necessary to protect circuits, equipment, and people from shocks and short circuits as well as to make accurate measurements. Isolation, also referred to as galvanic isolation, means no direct conduction path exists for the current to flow; no physical connection exists. Isolation can be accomplished using electromagnetic, capacitive, or optical devices. While physically and electrically isolating the circuitry from unwanted currents, required signals and power need to be transferred across the separated circuits. To transfer signals, transformers use magnetic flux, capacitive isolators use differential voltage and optocouplers use light to bridge the gap. This article discusses the use of isolation transformers.

Why Isolation?
Isolation transformers are used to:
*protect users from faulty equipment
*enable safe and accurate measurements
*avoid ground loops
*physically separate one part of an electrical system from another

Special Purpose Isolation Transformers
Isolation transformers have been developed for specialized applications. Some examples are:
Pulse transformers: optimized for transmitting rectangular electrical pulses and provide electrical isolation for digital signals. These are used in computer networks.

Austin transformers: invented by Arthur O. Austin, these power the air-traffic obstacle lamps you see on antenna structures. If not isolated, the lighting circuitry on the antenna mast would conduct radio-frequency energy to ground. These transformers also completely isolate the building AC mains from the tower.

Instrument transformers: to supply precise voltage for meters and are used to safely isolate control circuitry from high voltages/currents. The primary winding of the transformer is connected to the high voltage/current circuit and the meter is connected to the secondary circuit much like the connections shown in Figure 3.

Note: Some transformers are made with only one winding which is tapped at different places on the winding to divide it into primary and secondary portions. Known as auto-transformers, these devices do not provide isolation, as the single winding is shared. Isolation transformers have separate coils, with no physical connection between the coils, no earth ground.