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Buchholz Relay Explained: Gas, Alarms, Trips and Tests

Every oil-filled power transformer with a conservator carries a quiet guardian on the pipe between its main tank and the conservator: the Buchholz relay. It is one of the oldest protection devices in the substation, and it remains one of the most valuable, because it watches something no electrical relay can see directly — gas being produced inside the transformer. When insulation overheats, when oil breaks down under electrical stress, or when an internal fault releases energy, gas is generated. That gas rises through the oil, collects in the Buchholz relay, and the relay turns that physical evidence into an alarm, or, in severe cases, a trip. Understanding how it does that, and what its signals mean, is essential for anyone who operates, maintains or commissions oil-filled transformers.

This guide explains the Buchholz relay in plain language: its construction, its two operating stages, what gas in the relay actually means, how it is tested, and the mistakes that cause false alarms or missed faults. As with everything on this site, always work to your project specification, the manufacturer's instructions and your site safety rules; the general guidance here never overrides those documents. Work on or near an energised transformer, and any work that opens an oil circuit, is carried out only by competent persons under permit. If you are new to transformer testing as a whole, start with our power transformer testing and commissioning guide, which puts protection devices in context alongside the electrical tests.

Where the Buchholz relay sits and why that position matters

The Buchholz relay is installed in the pipe that connects the main transformer tank to the conservator, the expansion vessel mounted above the tank. Oil in the main tank expands and contracts with temperature, and the conservator gives it somewhere to go; oil therefore flows gently back and forth through that pipe as the transformer heats and cools. Any gas generated inside the main tank is lighter than oil, so it rises, and the natural path upward leads through that same pipe — straight into the relay chamber. That is the whole genius of the device: it sits exactly where gas must pass, and where a violent internal fault must push a surge of oil.

Because of this position, the pipework itself is part of the protection. The relay must be mounted with the correct inclination toward the conservator so that gas travels to it reliably, and the pipe must be free of pockets where gas could collect elsewhere and never reach the relay. During installation and commissioning, this is checked physically: the relay's orientation, the pipe slope, the oil level in the conservator, and the absence of trapped air after filling. A relay installed level, or on a pipe that sags, may never see the gas it is supposed to detect. It is a simple device, but it only works when its installation is right, which is why commissioning checklists treat the Buchholz pipework as seriously as the relay itself.

Inside the relay: two floats, two stages

Most Buchholz relays contain two hinged floats inside an oil-filled chamber, one above the other, each operating a switch contact. The upper float sits near the top of the chamber. In normal operation the chamber is full of oil and both floats are held up by buoyancy. When gas enters the chamber from the transformer, it displaces oil at the top, the oil level in the chamber falls, and the upper float drops. That movement closes the alarm contact. The alarm therefore means one thing, precisely: gas has accumulated in the relay chamber. It does not, by itself, prove a fault — but it demands attention, sampling and investigation.

The lower float operates the trip stage, and it can be moved in two ways. If gas continues to accumulate, the oil level falls further until the lower float also drops, closing the trip contact. More importantly, a sudden internal fault — an arc or a severe short circuit inside the tank — creates a violent pressure wave that drives a surge of oil through the pipe toward the conservator. That surge pushes a flap or vane in the relay, or drives the lower float down directly, depending on the design, and closes the trip contact within a fraction of a second. The trip stage therefore responds to speed and violence, not just to quantity: a slow accumulation alarms, a sudden surge trips. Many relays also include a test facility, typically a push-button or a small pump arrangement, that lets the floats be moved deliberately so the alarm and trip circuits can be proved without creating a real fault.

What gas in the relay actually means

Not all relay gas is bad news, and that distinction is where experience matters. Air can enter the oil system during filling, after maintenance, or when oil is processed, and small amounts of trapped air can collect in the relay in the days after such work. That gas is usually colourless, odourless air, and it stops accumulating once the trapped air has worked its way out. The correct response is to sample it, record the volume and the date, and watch whether accumulation continues. A one-off small collection after maintenance, followed by nothing, is a very different story from gas that keeps coming.

Gas produced by an internal fault behaves differently. Overheating of oil or paper generates gases such as hydrogen, methane, ethane and ethylene; arcing generates acetylene. Combustible gas, gas with a distinct smell, coloured gas, or gas that accumulates steadily or in increasing volumes all point to something happening inside the tank. The definitive step is laboratory analysis of a gas sample taken from the relay's sampling valve, combined with a dissolved gas analysis of the transformer oil itself. Together they usually identify whether the source is overheating, partial discharge or arcing, and how urgent the situation is. The rule of thumb is simple and worth memorising: a single small air collection after maintenance is noted and watched; any combustible gas, or any gas that keeps accumulating, is treated as a developing fault until analysis proves otherwise.

Testing the Buchholz relay

Testing falls into two parts: proving the relay's contacts and circuits, and proving the relay itself. The contacts and circuits are checked by secondary injection or by operating the relay's test facility, confirming that the alarm contact raises the correct alarm in the control room and that the trip contact operates the correct tripping circuit, including any lockout relay. This is routine protection testing, carried out under permit with the transformer tripping circuits made safe or the transformer already out of service, exactly as for any other protection relay. Records should show which contacts were proved, the trip circuit operated, and the reset state afterwards.

The relay mechanism itself is checked during outages: the floats move freely, the chamber is clean, the sampling valve and test facility work, and the pipework inclination and oil level are correct. A relay that has operated should never simply be reset and forgotten. The operating record — alarm or trip, gas volume, sample results, and the investigation outcome — is part of the transformer's history, and resetting without investigation destroys evidence. One common commissioning error is worth calling out: after oil filling, the relay chamber must be vented and filled properly, and any air deliberately introduced during filling work must be cleared, or the relay will alarm on the first day in service and train operators to ignore it. A protection device that cries wolf is almost as dangerous as one that never operates.

False alarms, missed faults and common mistakes

False alarms have several well-known causes. Air trapped during filling or maintenance is the most common. Vibration from a nearby fault or from switching can occasionally disturb a float, although modern relays are designed to resist this. A falling conservator oil level — after a leak, or after oil is drawn off for sampling without topping up — can expose the relay chamber and drop a float. Extremely cold weather can shrink oil volume enough to matter on a conservator that was filled low. Each of these has a signature: air after maintenance stops accumulating, vibration gives a single momentary operation with no gas, and a low oil level is visible on the conservator gauge. The investigation always starts with those simple checks before assuming an internal fault.

Missed faults are rarer but more serious, and they usually trace back to installation or maintenance rather than to the relay design. A relay mounted without the correct inclination, a pipe with a gas pocket elsewhere, a sampling valve left closed after work, or a relay chamber that was never properly filled after an overhaul can all blind the device. Another subtle cause is a conservator fitted with an air-cell or bladder that has failed, changing how oil and gas move through the system. The defence is boring but effective: follow the manufacturer's installation instructions exactly, check inclination and venting at commissioning, include the Buchholz pipework in outage inspections, and never reset an operation without recording it and investigating it. The Buchholz relay has protected transformers for nearly a century precisely because it is simple — and simplicity only works when the simple things are done right.

How Buchholz fits with the rest of transformer protection

The Buchholz relay is not a standalone protection scheme; it is one layer in a coordinated system. Electrical protection — differential, restricted earth fault and overcurrent, covered in our transformer protection guide — detects faults from currents and trips fast. The Buchholz relay detects the physical products of faults, including slow-developing overheating that may never produce a large enough current imbalance to operate a differential relay until it is too late. Pressure relief devices guard against tank rupture from the pressure a severe fault creates. Temperature devices guard against overloading. Each layer watches a different symptom, and together they cover faults that any single device would miss.

For the same reason, a Buchholz alarm is never investigated in isolation. The standard response combines it with oil and gas analysis (see our transformer oil testing guide), winding and insulation tests, a check of load and temperature history, and a review of recent maintenance — the same evidence base used when maintaining a transformer or deciding whether it needs overhauling. And because cooling affects everything — a transformer running hot generates gas faster — the cooling system is always part of the investigation too. The relay tells you something is happening; the rest of the evidence tells you what.

Frequently asked questions

Related guides: power transformer testing and commissioning guide, transformer protection explained, transformer cooling systems, transformer construction and types.