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.
- QJ1-50
- QJ4-50
- QJ13-50TH
- QJ1-80
- QJ4-80
- QJ13-80ATH
- QJ2-40
- QJ3-40
- QJM-40
* Values marked TBD are confirmed per order against factory datasheets. Full installation dimension chart available as PDF on request.
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 | Pipe nominal diameter | Design |
|---|---|---|
| QJ1-50 / QJ4-50 / QJ13-50TH | DN 50 mm | Double-float, alarm + trip |
| QJ1-80 / QJ4-80 / QJ13-80ATH | DN 80 mm | Double-float, alarm + trip |
| QJ2-40 / QJ3-40 | DN 40 mm | Single-float / sealed |
| QJM-40 | DN 40 mm | Sealed (fully enclosed) type |
| Gas volume to alarm / trip speed / contact capacity | TBD* — per factory datasheet, PDF on request | |
* To be confirmed per order. We do not publish unverified data — request the official datasheet with your inquiry.
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.