How does an electrosurgical unit work, and why does it cut and coagulate instead of shocking the patient? An electrosurgical generator converts mains power into high-frequency alternating current, shapes it into a chosen waveform and delivers it through an active electrode, where concentrated current density heats tissue. This article explains the working principle, the role of frequency and waveform, and the safety circuits that biomedical engineers and OR staff should understand.
What is an electrosurgical generator?
An electrosurgical generator is the power source of an ESU. Internally it rectifies mains voltage, creates direct current and feeds a high-frequency oscillator and amplifier. The output stage is isolated from earth and from the mains, so the patient is not part of a path to ground except through the intended circuit. A microprocessor controls output according to the mode and the settings.
The generator is only part of the system. The active electrode, in a pencil or an instrument, concentrates the current, and the patient return electrode or the second tip of a bipolar instrument completes the circuit. See diathermy machine parts for a full parts list.
Electrosurgical generator frequency and why it matters
Electrosurgical generator frequency is in the range of hundreds of kilohertz to a few megahertz. Below roughly 100 kHz, alternating current stimulates nerves and muscles, which is dangerous for the heart; at higher frequencies the tissue is heated without such stimulation. For this reason mains-frequency current is never used for cutting.
Typical units operate at several hundred kilohertz. The BOWA ARC 400 specifies an output frequency of 350 kHz, and the ARC 350 lists 350 kHz and 1 MHz. Higher frequencies can reduce leakage in some situations, but the clinical effect is controlled mainly by voltage, power and waveform.
Waveforms: cut, coagulation and blend
Tissue effect depends on power density and on how the energy is delivered over time. A continuous low-voltage waveform gives a clean cut by vaporising tissue along the electrode. An interrupted waveform with higher peak voltage delivers bursts of energy, producing coagulation. Blend modes mix the two. Modern generators add controlled-arc and sealing modes, which regulate voltage and current automatically.
Read cut, coag and blend waveforms to see how peak voltage and duty cycle shape the effect. The generator measures the tissue impedance and adjusts output so that the set power is delivered at the stated load, which is why specifications quote power at defined resistances such as 200 Ω for monopolar and 75 Ω for bipolar.
- Cut: continuous waveform, lower voltage
- Coag: interrupted waveform, higher peak voltage
- Blend: mixes cutting with haemostasis
- Sealing modes: regulated bipolar energy with feedback
Monopolar and bipolar circuits
In monopolar mode current flows from the active electrode through the patient to a large neutral pad, where the low current density prevents heating. In bipolar mode current passes only between two tips of an instrument. These circuits and their safety implications are compared in monopolar vs bipolar electrosurgery.
The BOWA ARC 400 supports monopolar and bipolar cutting, coagulation and sealing in one unit, with up to 400 W monopolar (200 Ω) and 400 W bipolar (75 Ω). The Richard Wolf HF-GENERATOR 2260 provides instrument-specific presets, and touch navigation keeps the voltage within the limit permitted for the connected instrument.
Safety circuits and maintenance
A modern generator monitors itself. Continuous self-testing, neutral-electrode contact monitoring, arc control and instrument recognition protect the patient and the equipment. The ARC 350 lists integrated arc control, continuous self-testing and neutral-electrode monitoring. The relevant product standard is IEC 60601-2-2, together with IEC 60601-1; see IEC 60601-2-2 explained and return electrode contact monitoring.
Maintenance includes visual inspection, output verification with a test device, cable and footswitch checks and software updates as the manufacturer prescribes; see electrosurgical unit maintenance. Beckfinity's specialists can help with training and service planning: request a quote.
Frequently asked questions
Why does electrosurgery not cause electric shock?
Because it uses current at hundreds of kilohertz, well above the frequencies that stimulate nerves and muscles, and the isolated output limits unintended paths.
What frequency do electrosurgical units use?
Typically a few hundred kilohertz up to about 1 MHz or more. For example, BOWA ARC 400 uses 350 kHz.
What is the difference between cut and coag?
Cut uses a continuous, lower-voltage waveform; coag uses an interrupted, higher-voltage waveform that heats tissue without vaporising it.
How is a diathermy machine tested?
With periodic visual checks, output measurement using a suitable analyser, and self-tests at start-up, as specified by the manufacturer.
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