Buchholz Gas Relay — QJ Series
Double-float gas relay for oil-immersed power transformers. Detects gas accumulation from internal faults (first-stage alarm) and oil surge during severe faults (second-stage trip), protecting the transformer before catastrophic failure.
- QJ4-25
- QJ4-50
- QJ4-80
- QJ1-50
- QJ1-80
- QJ13-50
- QJ13-80
- QJ2-40
- QJM-40
- QJ35S-25 / 50 / 80
Full installation dimension chart available as PDF on request. Where the specification calls for a sealed / fully enclosed housing, see the sealed gas relay page.
Product Photos
Straight from the production line — square-flange, round-flange, double-float and digital variants.
How It Works
Two floats, two levels of protection — the logic every substation engineer knows.
Gas accumulation → Alarm
Minor internal faults (partial discharge, arcing in oil) generate gas that collects in the relay housing, lowering the oil level and triggering the upper float alarm contact.
Oil surge → Trip
Severe faults push an oil wave through the relay; the lower float (baffle) trips the breaker circuit to isolate the transformer.
Gas sampling
A test/sampling valve on top allows gas extraction for dissolved-gas analysis. Matching gas sampling devices available.
Technical Data
The table below is completed with factory-confirmed values for every order. Ask for the dimensional PDF.
| Model group | Pipe nominal diameter | Design |
|---|---|---|
| QJ1-50 / QJ4-50 / QJ13-50 | DN 50 mm | Double-float, alarm + trip |
| QJ1-80 / QJ4-80 / QJ13-80 | DN 80 mm | Double-float, alarm + trip |
| QJ4-25 / QJ6-25 / QJ7-25 / QJ13-25 / QJ18-25 | DN 25 mm | Small-frame double-float |
| QJ2-40 / QJ3-40 | DN 40 mm | Single-float / sealed |
| QJM-40 | DN 40 mm | Sealed (fully enclosed) type |
| QJ35S-25 / QJ35S-50 / QJ35S-80 | DN 25 / 50 / 80 mm | Double-float, extended frame |
General parameters: factory standard Q/TTJ 32.07.11-2018, rated for oil-immersed transformers of 800 kVA and above. Gas-volume alarm thresholds, trip speed and contact ratings are stated on the individual factory datasheet — we do not publish unverified figures. Request it with your inquiry.
Compliance
Documents are matched to the models and the destination country on your order.
Test and conformity documents applicable to the models you buy are supplied as copies with your quotation and again with the shipping documents. Tell us the destination country and any standard you must satisfy when you enquire — we confirm what can be supplied before you commit.
Document availability is confirmed per model and destination before you order. See how we handle quality & compliance →
Frequently Asked
Can you supply relays matching our existing transformer brand?
Yes. Send us the relay nameplate or the old unit's photos and pipe diameter — we match flange dimensions and float settings to your installation.
Do you offer single-float or sealed versions?
Yes — QJ2-40 and QJ3-40 single-float types, plus the fully sealed QJM-40. Double-float QJ1/QJ4/QJ13 remain the standard for conservator-type transformers.
What documents come with the shipment?
Packing list, commercial invoice, inspection record. Test reports and certificates from the manufacturing partner are available on request.
Need QJ4-50, QJ4-80 or a custom size?
Tell us the pipe diameter and quantity — priced offer within 24 hours.