Buchholz Relay Selection Guide (QJ Series)

The Buchholz relay — often called a gas relay — is the primary protection device inside the conservator pipe of every oil-immersed transformer. This guide covers how to pick the right one without over-specifying.

1. What a Buchholz relay does

It detects two fault conditions: slow gas accumulation from minor faults (alarm), and sudden oil surge from major faults (trip). A double-float design covers both. It is a safety device, so selection is about fit and reliability, not features.

2. The three things that decide the model

Factor	Why it matters
Conservator pipe bore (Φ)	The relay body must match the pipe diameter — typically TBD* mm for the QJ range. This is the first question we ask.
Transformer capacity	Larger units need larger gas-volume settings and sometimes a different body size.
Mounting orientation	Pipe slope and flange type must match the existing conservator connection.

3. QJ series at a glance

The QJ range covers the common oil-immersed transformer sizes. Exact model-to-capacity mapping and action-volume settings are confirmed per order and listed on the Buchholz relay product page.

• QJ4-25 — widely used on mid-size distribution and power transformers.
• QJ4-50 — larger bore for bigger units.
• Double-float and fully-sealed variants available for specific applications.

Model availability and dimension tables are being finalised with our manufacturing partner; figures shown here describe the typical QJ family, not a guaranteed stock list. Confirm the exact part on inquiry.

4. ONAN vs. forced cooling

A Buchholz relay is required on all oil-immersed transformers with a conservator, regardless of cooling class. Forced-cooled (ONAF/OFAF) units additionally need an oil flow relay on the cooling circuit — the Buchholz relay alone does not cover oil-flow loss.

5. Common mistakes to avoid

• Ordering by "the old part number only" without the pipe bore — the bore decides fit.
• Forgetting the gas-volume setting — mismatch can cause nuisance alarms or missed trips.
• Mixing 50 Hz and 60 Hz export requirements on accessories with moving parts.

FAQ