A Buchholz relay is the gas- and oil-actuated device mounted in the pipe between a liquid-filled transformer's tank and its conservator. Slow gas buildup from an incipient fault drives an alarm; a fast oil surge from a severe fault trips the unit off the system. It catches winding turn-to-turn faults that current differential protection can miss, and it is one of the standard main protections on conservator-type oil transformers.

1. What Buchholz protection actually is

"Gas protection"  is the Chinese name for what the rest of the world calls Buchholz protection, after the engineer who patented the relay. It is a protection mounted on the pipe that connects the main tank to the oil conservator — the small expansion tank above the transformer. Inside the relay are two independent elements:

• An upper element that responds to gas accumulating at the top of the relay (the alarm stage).
• A lower element that responds to a rapid flow of oil through the pipe (the trip stage).

Both elements watch the same pipe, but they answer different questions: is something slowly breaking down inside, or has something just failed hard?

2. Where the relay sits

The relay lives in the middle of the connecting pipe between the transformer tank and the conservator, never at the bottom of the tank and never inside the conservator itself. That placement is deliberate. Gas from an internal fault rises through the oil and migrates up the pipe; oil moving under a sudden pressure wave passes through the same pipe on its way to the conservator. Putting the relay at the pipe's midpoint is what lets one device see both the gas and the surge.

That placement also defines the first hard limit: a Buchholz relay needs a conservator and a flow path between tank and conservator. A hermetically sealed, conservator-less design (the "M" sealed units) has nowhere for the gas to collect or the oil to surge toward, so it uses a pressure-relief valve and a sudden-pressure relay instead. This is also why dry-type units carry no Buchholz at all — see the oil vs dry-type comparison.

3. The alarm stage (light gas)

When insulation breaks down, it releases gas that bubbles up into the relay and displaces oil. As the oil level inside the relay falls, a float drops and closes a contact — a normally open contact that, once closed, sends an alarm to the control room. No breaker opens. The purpose is to warn an operator that something is degrading before it turns into a trip.

Older relays carried a mercury switch on that float; modern units use a magnetic float and a reed contact, which avoids the mercury. Either way, the trigger is gas volume, not speed.

4. The trip stage (heavy gas / oil surge)

A severe fault — an arcing winding fault, for example — boils the oil fast enough to send a slug of oil and gas racing up the pipe. The relay's lower element has a flap or baffle in the oil path. When the oil velocity reaches roughly 0.6–1 m/s (about 2–3 ft/s), the surge pushes the flap, a reed contact closes, and the relay issues a trip that disconnects the transformer from all connected sources.

The velocity setting is a trade-off: too low and pump or load swings cause nuisance trips, too high and a fast-developing fault gets a late trip. Most applications land near 1 m/s.

5. What it catches that differential misses

Buchholz earns its place because it is sensitive to incipient, low-current faults — especially winding turn-to-turn short circuits with only a few turns involved. A small number of shorted turns may not change the line currents enough for differential protection to see, but it still breaks down insulation and makes gas. That gap is what Buchholz fills, and it is why it is treated as a main protection rather than an add-on.

It works alongside differential protection, not instead of it. Differential clears the high-current faults fast; Buchholz catches the slow, low-current ones and backs up differential if that scheme fails to operate.

6. When it is required

The source cites Chinese design rules that require Buchholz on oil-immersed transformers at 800 kVA and above, and on 400 kVA and above units installed indoors (workshops and similar). Those thresholds come from GB 50062 and GB 50053, not from any U.S. document.

In North America there is no single NEC line that says "install a Buchholz relay at 800 kVA." The relay is simply standard practice on conservator-type liquid-filled transformers, typically from a few hundred kVA upward, and the exact threshold on a given project follows the utility's protection requirements and the coordination study. Our liquid-filled range, including the sealed designs that use different protection, is on the products page.

7. Installation slope requirements

For the relay to do its job, gas has to be able to travel from the tank up to the relay instead of getting trapped. That drives two installation slopes:

• Transformer top cover to horizontal: about 1%–1.5%.
• Connecting pipe (tank to conservator): about 2%–4%, rising toward the conservator.

Get the slope wrong and gas pools in the pipe instead of reaching the relay, which silently defeats the alarm stage.

8. What happens on operation and how to read the gas

On a light-gas alarm, the transformer keeps running. The alarm is a cue to sample and inspect — check dissolved-gas trends, look for overheating, and confirm there is no genuine fault before resetting. Do not just reset and walk away.

On a heavy-gas trip, the relay cuts all power sources to the transformer to stop the fault from propagating. Treat it as a real event and investigate before re-energizing.

After any operation, the gas collected at the relay's upper vent is the fastest clue to what happened. Whether the gas dissolves in oil, its color, and how fast it accumulates all point to different fault types, and that reading drives the repair plan. Buchholz function and gas sampling are part of the checks in the routine testing guide. More installation and standards notes sit in the resources library, and field examples of these relays in service are in our projects archive.

9. FAQ

Does a Buchholz relay work on a sealed, conservator-less transformer?

No. The relay needs a conservator and a flow path between tank and conservator so gas can collect and oil can surge toward it. Hermetically sealed units (the "M" designs) have neither, so they rely on a pressure-relief valve and a sudden-pressure relay instead. Dry-type transformers have no oil at all, so no Buchholz either.

What is the difference between the alarm and trip stages?

The alarm (upper) element reacts to gas volume building up slowly from incipient faults and only signals — it does not open a breaker. The trip (lower) element reacts to a fast oil surge from a severe fault and disconnects the transformer. They share the same pipe but trigger on different physical events.

What oil velocity trips the relay?

The source design trips around 0.6–1 m/s (roughly 2–3 ft/s). Many relays are adjustable, and most applications settle near 1 m/s. Confirm the setting against the relay model and the protection study. 

Is Buchholz protection required by the NEC?

Not by a specific NEC line. The 800 kVA / 400 kVA thresholds in the source are Chinese code (GB 50062 / GB 50053). In North America the relay is standard on conservator-type liquid-filled units, with the exact threshold set by the utility and the protection coordination study. 

Can Buchholz detect a winding turn-to-turn fault?

Yes, and that is its main strength. A fault involving only a few shorted turns may not shift line currents enough for differential protection to see, but it still decomposes insulation and makes gas — exactly what the alarm element picks up.

Why does the connecting pipe need a slope?

So gas released in the tank migrates up to the relay instead of getting trapped in a low spot of the pipe. The source calls for a 1%–1.5% top-cover slope and a 2%–4% pipe slope toward the conservator; confirm against the relay maker's instructions.

What does the collected gas tell you about the fault?

Color, whether it dissolves in oil, and how fast it accumulates all distinguish fault types — from harmless entrained air to genuine arcing. Sampling that gas is usually the first step after any operation and shapes the repair plan.

What should I do when the light-gas alarm comes in?

Treat it as a warning, not a trip. Keep the unit running if it is stable, pull dissolved-gas and oil samples, look for overheating or progressive trends, and only reset after you have confirmed there is no developing fault.