Transformer Isolation

Electrical isolation is necessary to protect circuits, equipment, and people from shocks and short circuits.

Recommended Level

Beginner

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
Let's look at setups where isolation is needed and how it can be provided using an isolation transformer:
Figure 1. shows how a measurement might be taken across Z1 at Test Points TP1 and TP2, attempting to measure the voltage across impedance Z1.  TP1 and TP2 are part of the generator circuit; the generator ground, the oscilloscope ground and oscilloscope probe ground are all in common.   The cable shield (ground) of the oscilloscope probe is tied to ground via the oscilloscope's chassis (which you can verify with an ohmmeter.)  With the oscilloscope probe connected at TP1 and the oscilloscope probe ground connected at TP2 as shown, Z2 is shorted out of the circuit when the probe ground provides an alternate path to ground. This means 1) the measurement of v1 is not accurate, and 2) if Z2 was the current limiting impedance, the current through Z1 could rise to a dangerous level and damage the circuit.  A person, standing on a grounded floor, accidentally touching the circuit at TP2 would have the same shorting effect (and feel it).

Fig. 1 Grounding at aTest Point

Fig. 2 Isolated Test Point

The circuit in Figure 2 uses an isolation transformer.  Powered through the isolation transformer, the circuit with Z1 and Z2 no longer shares earth ground with the generator and oscilloscope.  Now connecting the test probe at TP1 and the probe ground at TP2 does not complete a circuit and voltage v1 can be measured accurately.  An isolated circuit is a live circuit and when using grounded probes, you still need to be aware of the circuit(s) you are working on and that the probes are not connected in a way that creates a ground loop within the isolated circuit.
In plumbing, you sometimes hear of hot water coming out of cold water faucets, even though there's not supposed to be a connection.  Somewhere in the water pipe connections, there was a common point where a crossover happened.  The same surprising results can happen in an electric circuit where an unintentional ground is introduced.  It's not supposed to happen, but a common point to ground was introduced.  Knowing the circuit, using isolation transformers where there a ground could be  introduced, following safe working procedures, all work to reduce unexpected results.
Sometimes the term “isolation transformer” is applied to transformers that isolate AC spikes, transients and noise, but maintain an earth ground connection. This type of transformer would not provide electrical isolation.  You should verify the transformer you are using does provide electrically isolated outputs and doesn't supply an earth ground;  check there is no continuity between the primary and secondary.  If the transformer has a inter-winding screen, the screen should be connected to earth ground; also it's common practice to connect the transformer frame to earth ground as well.
When equipment under test is powered by an isolating transformer, its ground (equipment ground) is separated from earth ground; the transformer isolates the device under test from the common supply ground.  A person working on the equipment (standing at earth ground) can't accidentally provide a path to ground should they make contact with the circuit. This makes the set up safer for users, eliminating the possibility of a shock. Should they touch a live part of the circuit by accident, there's no conductive connection to earth ground.

Before ground fault circuit interrupter (GFCI) receptacles became 'code', consumer products included isolation transformers and hotels had 'razors-only' sockets incorporating an isolation transformer. The razor-only socket provided protection if a razor fell into the water or if someone touched a conducting surface (like a wet faucet) while holding it. The isolation transformer in the socket prevented current flow thru the user's body.
An isolation transformer can also be used to physically separate portions of an electrical system. It would be dangerous to try to meter high voltage lines, where the voltages could be above 30,000 V. You would risk making contact while trying to connect a measuring device.  By including an isolation transformer as part of the design, the voltage can be stepped down to a lower voltage in the meter's range, as shown in Figure 3.

Fig. 3 Step-Down Transformer used to meter High Voltage Line

In this case, a step-down isolation transformer is needed. The step-down ratio is determined by the formula:

Ep(volts)Es(volts)=NpNs

where, Ep is the primary voltage
           Es is the secondary voltage
           Np is the number of turns in the primary
           Ns the number of turns in the secondary
Note:

NpNs=a

, the transformation ratio.
If the voltage was 30,000 volts, a step-down transformer with a = 300 would give a voltage of 100 volts, which could be safely measured.

Isolation Transformer Construction

Transformers can be described as two coils surrounding a core of ferromagnetic material, as shown in Figure 4.

Fig. 4 Transformer

The schematic representation shows the primary and secondary coils; the electric source is connected to the primary, the isolated output is taken from the secondary. The coils are physically separate from each other and the core.  Michael Faraday first used an early transformer during his experiments investigating electromagnetism. Faraday found that a wire carrying a current induces a magnetic field surrounding the wire and that when two separate wires were coiled around a toroid of soft iron, a current in one induced a magnetic field, and the changing flux in turn induced a voltage in the other.  Now known as mutual induction, Faraday is credited with discovering that an electromotive force is induced in a circuit by a changing magnetic flux according to the formula:    

E=−dΦBdt

.    Sometimes this is shown using the absolute value of E:

|E|=dΦBdt

. The negative indicating the electromotive force opposes the current.
Because Faraday was working with DC voltage, he only saw the effect of electromagnetic induction when a battery was initially connected or disconnected to the circuits, when the magnetic flux was changing.  With AC power connected to the primary, the varying current creates a varying magnetic field, the magnetic flux is realized in the core, and that in turn induces a voltage the secondary, with no electrical path between the two coils. The inductive coupling provided by the changing magnetic flux between the two coils allows communication across transformer. The magnetic field induced by a transformer depends on the number of turns/unit length of the windings, the permittivity of the magnetic core, and the current magnitude. The first commercially viable transformer was invented by William Stanley, working for George Westinghouse in the 1880s.
Although any transformer consisting of two separate coils and no grounding shields provide isolation, the term isolation transformer applies to transformers especially designed for the purpose of providing electrical isolation; whose primary purpose is to isolate an AC source from circuits, devices and equipment.  The design of an isolation transformer takes into account anything that may couple the primary and secondary windings. They often have special insulation between the primary and secondary coils, and are specified to withstand high voltage between windings. Because power line/transient voltage noise can be coupled thru the capacitance and resistive paths of the coils, isolation transformers have additional features to reduce common-mode noise (occurring on both the hot and neutral wires referenced to ground), transverse mode noise ( occurring between the hot and neutral wires) and electromagnetic noise. DC signals are blocked by the transformer as well as interference caused by ground loops. For sensitive equipment ( computers or measuring instruments ) electrostatic shields are included to reduce any capacitance between the windings.
Isolation transformers used for safety usually have a turns ration of 1:1, with the number turns in the primary and secondary windings equal, but step-up and step-down isolation transformers are used when the voltage also needs to be changed.  When choosing an isolation transformer, check the specs for the features included, the ratings and how they are constructed.

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 an 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.

Safety Always

Isolation transformers make working on AC equipment safer and can protect against unintentionally introducing shorts in the circuit. Working on the principle of mutual induction, they are used to break ground loops and remove unintended current paths where accidental contact could cause problems.  When choosing an isolation transformer, select one with appropriate ratings and specs for your requirements.
The isolated circuit is still a live circuit!  When using an isolation transformer, whether powering the unit under test or an oscilloscope or other equipment, knowing the ground(s) in use; checking voltages and current in your work area and your circuit, following all safety measures, are still required!