17.3 Transformer Key Safety Issues

Transformer Key Safety Issues

Quick Answer: The single most important transformer safety rule for an electrician: never open-circuit a current transformer secondary while primary current is flowing — short the secondary first. The mirror rule: never short-circuit a voltage transformer secondary. Beyond that, transformers store energy, can be back-fed, and their secondary earthing arrangement determines whether separation is real.

Transformer Types You Will Meet

The autotransformer entry is a favourite exam distinction: because the windings are electrically common, an autotransformer gives no isolation and cannot be used where separation is the protective measure.

Current Transformers — the Critical Rule

A CT is a current source. Its primary current is fixed by the load in the main circuit, and the core flux is normally kept low because the secondary current opposes it.

Open the secondary while primary current flows and the opposing ampere-turns vanish. The core saturates hard on every half cycle, and the collapsing flux induces an extremely high voltage across the open terminals — potentially thousands of volts. That voltage can:

• kill or seriously injure anyone in contact with the secondary terminals;
• break down the CT insulation, destroying the transformer;
• start a fire in the metering cubicle.

Safe procedure:

1. Fit and confirm the shorting link (or shorting terminal block) across the CT secondary.
2. Only then disconnect the meter, relay or wiring.
3. Reconnect the burden, confirm connections, then remove the shorting link last.
4. Never leave a CT secondary unterminated at the end of a job.
5. Earth one point of the secondary circuit where required, so the secondary cannot float to primary potential through capacitive coupling.

Where CT secondaries are wired to a switchboard, expect to find a dedicated shorting terminal block — know how to operate it and say so on the practical.

Voltage Transformers — the Opposite Rule

A VT is a voltage source with a small burden. Short-circuiting the secondary draws very large current, destroying the VT and potentially causing a primary-side fault. VT secondaries are therefore protected by fuses or a miniature circuit breaker.

Additional VT hazards:

• Back-feed. Applying voltage to a VT secondary energises the primary at full HV. Isolate and prove dead on both sides before working.
• Secondary earthing is provided so a primary-to-secondary insulation failure cannot raise the secondary to HV potential.

Isolating and Safety Isolating Transformers

An isolating transformer separates the secondary circuit from earth and from the supply. That separation is the protective measure, so:

• The secondary must not be earthed if electrical separation is intended.
• Normally only one item of equipment is supplied from an electrically separated circuit, unless the specific requirements for supplying more than one item are met.
• Exposed conductive parts of the separated circuit are bonded together but not connected to protective earth or to other circuits.

A safety isolating transformer is the source used for SELV and PELV circuits (Chapter 8). Substituting an ordinary transformer, or an autotransformer, defeats the entire protective measure — a serious defect.

Safe Working Procedures for Connection and Testing

1. Isolate and prove dead on every side. A transformer has at least two circuits; isolating one leaves the other capable of back-feeding.
2. Discharge and earth where the installation or manufacturer’s procedure requires it — large windings and any associated capacitors store energy.
3. Check the vector group and polarity before paralleling; mismatched connections produce circulating current.
4. Confirm tap position and record it before and after any change.
5. Insulation resistance test primary to secondary, primary to earth and secondary to earth, using the voltage the manufacturer specifies.
6. Verify the enclosure earth and the correct sizing of the protective earthing conductor.
7. Provide ventilation and clearance as specified; a transformer derated by a blocked enclosure will run hot and fail early.

AS/NZS 3000 Requirements and Restrictions

The Wiring Rules place requirements on the installation and use of transformers, including:

• Where they may be located and the accessibility and ventilation they require.
• Protection of the transformer and of the circuits on each side.
• The conditions attaching to electrical separation, SELV and PELV sources.
• Restrictions on using an autotransformer where isolation or separation is required.

On a written item, name the hazard, name the control, and name the standard family — for example: "CT secondary open circuit produces lethal induced voltage; control is to apply the shorting link before disconnecting the burden; installation and protection requirements are in AS/NZS 3000."