Safe bench work: mains, stored energy and high voltage

Set up a repair bench and a working routine that keep you, the instruments and the board safe: isolation, ESD control, capacitor discharge, high-voltage rules, safe live measurement, and the decision not to work live at all.

Why bench safety is a skill of its own

Medical equipment puts several kinds of danger in one box. A patient monitor has mains on its power supply primary, a few hundred volts DC on the bulk capacitor, and on older models a backlight inverter that makes well over a kilovolt. An ultrasound system has a high-voltage transmit supply for the probe. A defibrillator stores enough energy in its capacitor to stop a heart, which is exactly what it is built to do. An X-ray generator works with tens of kilovolts in an oil tank and keeps a charge in its cables and capacitors after it is switched off. An infusion pump or a ventilator looks harmless until you remember the mains-side power supply and the battery that can deliver a large short-circuit current.

None of this is exotic, and that is the danger. Most accidents on a repair bench happen to experienced people doing a routine job in a hurry: a probe slips across two pins, a capacitor that was assumed to be discharged is not, a scope ground clip goes onto the hot side of a power supply. The rules in this lesson exist to make the safe way the automatic way, so that a tired engineer at the end of the day still does it right.

Two principles run through everything. First, work dead whenever you can: most diagnosis can be done with the unit disconnected, discharged and measured with an ohmmeter or a current-limited bench supply. Second, when you must work live, decide in advance exactly what you will measure, where you will connect, and how you will get your hands away, before you switch on.

The isolation transformer: what it protects and what it does not

A bench isolation transformer has a secondary winding that is not connected to earth. On a normal mains supply one conductor is near earth potential, so touching the other conductor drives current through your body to earth. On the floating secondary of an isolation transformer there is no such return path, so a single point of contact with the circuit does not, by itself, drive current through you to earth. That is the whole benefit, and it is a real one, especially for work on the mains-referenced primary side of a switch-mode supply.

Understand its limits. It does not protect you if you touch two points of the circuit at once, because the full voltage between those points is still there. It does not reduce the voltage on the bulk capacitor or anywhere else inside the unit. It removes the protection an RCD or GFCI would give you, because no current flows to earth for the RCD to detect. Its secondary can gradually leak to earth through cable capacitance, faulty insulation or a second earthed instrument, and as soon as one point is earthed you are back to an ordinary supply without knowing it.

Use one transformer for one unit under test, rated for the load, with its secondary clearly labelled. Never power two devices from the same isolated secondary when you are probing one of them, and never connect anything to a patient through it. Never 'solve' a measurement problem by lifting the protective earth of the oscilloscope; that makes the oscilloscope case live if the ground clip touches a high-voltage node. The correct tools for floating measurements are covered in lesson 4.

ESD: the damage you do not see

A person walking across a floor can charge up to several thousand volts, and on carpet in dry air to tens of thousands. Most people only feel a discharge above roughly 3,000 V, so many discharges go unnoticed. Modern CMOS chips, MOSFET gates, LCD drivers and the front ends of ultrasound and monitoring boards can be damaged by much less, many of them by under 100 V. The damage is often latent: the board passes on the bench and fails weeks later in the ward, and nobody connects the two events.

An ESD-safe workstation has a dissipative mat bonded to a common earth point, a wrist strap with a built-in series resistor (typically 1 megohm, which limits current if you touch something live; this is the common industry value, not a figure you choose yourself) connected to the same point, and dissipative bags and trays for boards. Test the wrist strap regularly with a strap tester and record it. Handle boards by the edges, keep plastic, foam and paperwork away from the open board, and carry spare boards in their shielding bags until the moment you fit them. Return faulty boards to the supplier in proper ESD packaging too; an exchange board damaged in transit is still a damaged board.

Remember that the wrist strap is an ESD tool, not an electrical safety tool. Wrist strap manufacturers and the ESD Association handbook advise against wearing an earthed strap wherever you could touch exposed circuits of 250 V or more. When you work on a live mains or high-voltage circuit, take the strap off unless your workshop's written rule is stricter, and handle boards only with power off. A strap that bonds you to earth is the opposite of what you want if the unit is on an isolated supply and you are close to the hot side.

Stored energy: discharge, measure, then touch

The energy in a capacitor is half the capacitance times the voltage squared. A 470 microfarad bulk capacitor charged to 400 V holds about 38 joules, enough to burn, to make you jump into something worse, and to vaporise a screwdriver tip. Bleeder resistors are meant to discharge it after power-off, but bleeders fail open and many designs rely on the load to do the job. Never assume a capacitor is discharged because the unit has been unplugged for a while.

The routine is always the same. Disconnect the unit from the mains and from its battery. Wait the time the manufacturer specifies. Measure the voltage across the capacitor with a meter set to DC volts. If it is above a safe level, discharge it through a resistor of suitable value and power rating, held with insulated clips or a purpose-made discharge tool, never by shorting with a screwdriver. Measure again. A resistor in the kilohm range discharges a bulk capacitor in seconds; the time constant is resistance times capacitance, and after five time constants less than one percent of the voltage is left.

Large capacitors can recover some charge after discharge because of dielectric absorption. For high-voltage capacitors, leave a shorting link across the terminals while you work and remove it only before power-up. In X-ray generators and defibrillators follow the manufacturer's discharge procedure exactly, including the wait time and the use of an earthing stick where specified. These procedures exist because people have been hurt by energy that was supposed to be gone.

Measuring live circuits safely

When a measurement must be made live, the setup is done dead. Choose a meter and probes rated for the measurement category and voltage involved, with shrouded connectors and, ideally, fused leads. Attach hook clips or probe holders to the points you need with the unit disconnected and discharged, route the leads so that they cannot be pulled or touch anything, then step back and switch on. Read, switch off, discharge, and only then move the clips.

If you must probe by hand, keep one hand away from the unit, stand on a dry insulating surface, remove rings and watches, and wear eye protection near large electrolytics that can vent violently. Know which parts of the board are mains-referenced: in a switch-mode supply everything on the primary side of the transformer, including the heatsink of the switching transistor on many designs, can be at mains potential. Feed the unit through a variable or current-limited source when you can, so that a fault does not have the full mains behind it. Local rules say the same. The Kazakhstan safety rules for consumer electrical installations (clause 180) and the Russian labour safety rules for electrical installations (order 903n, clause 4.5) require this for live work up to 1000 V: guard nearby live parts, stand on an insulating stand or dielectric mat, use tools with insulated handles and dielectric gloves, and no short or rolled-up sleeves. Only staff whose employer has authorised them for live work under these rules may do it.

Never work alone on mains or high voltage. Someone in the room who knows where the isolating switch is and how to call for help is part of the safety system, not a formality.

When not to work live

Many repairs never need live measurement, and some must never be attempted live. Make the decision before you open the unit, not when you are already holding the probe.

Finally, the unit you work on must never be connected to a patient. Take it out of clinical use, label it, and return it only after the verification and electrical safety test described in lesson 10.

• X-ray generator high-voltage sections, tanks and cables: only per the manufacturer's procedure, with the training it requires.
• Defibrillator energy storage and charging circuits: manufacturer's procedure only.
• Backlight inverters and other kilovolt circuits: measure dead, or with proper high-voltage probes and a setup made dead.
• Any circuit you do not yet understand: read the schematic first (lesson 3).
• Any situation where you are tired, alone, rushed or missing the right instrument.
• Any unit that is still connected to a patient: never, under any circumstances.

On-site checklist

• Unit out of clinical use, labelled, disconnected from any patient.
• Mains and battery disconnected before opening.
• Wait time observed; capacitors measured, discharged through a resistor, measured again.
• Shorting link on high-voltage capacitors while working.
• ESD strap tested and worn for board handling; boards in shielding bags.
• Hot side and high-voltage areas identified on the schematic before power-up.
• Clips attached dead; one transformer per unit; someone else present for live work.

Check yourself