QJ12-25 Dual-Float Buchholz Relay

Gas-Actuated Relay with Dual Float Switches for Transformer Protection

Model: QJ12-25   |   Connection: DN25   |   Standard: IEC 60076-1

1. Product Introduction

The QJ12-25 is a dual-float Buchholz relay (gas-actuated relay) installed in the piping between the transformer tank and the conservator. It provides internal-fault protection by detecting two independent conditions: the gradual accumulation of gas generated by minor faults, and the sudden oil flow surge caused by severe arcing faults.

Unlike single-float designs that rely on a mechanical flap, the QJ12-25 uses two separate floats, each equipped with its own mercury-tilt switch. The upper float responds to gas accumulation and issues an alarm; the lower float responds to rapid oil displacement and triggers tripping. This dual-stage, float-only architecture offers high sensitivity, a simple structure, and reliable two-level protection without any oil-flow baffle.

Dual-Float Working Principle

• Normal operation: Both floats are immersed in oil and held in their upper positions; both alarm and trip contacts remain open.
• Minor / incipient fault (alarm): Decomposition gas collects at the top of the relay body, lowering the oil level. The upper float drops and its mercury switch closes to activate an alarm signal (e.g., at 200-250 cm³ of gas).
• Severe internal fault (trip): A large volume of gas and rapid oil flow forces the oil level down sharply. The lower float drops and its mercury switch closes to trip and disconnect the transformer (oil flow surge corresponding to roughly 0.8-1.2 m/s).

2. Model Designation

2.1 Model and Description

Code	Meaning
QJ	Gas-actuated relay (Buchholz relay)
12	Design series (dual-float type)
25	Connection diameter DN25 (pipe / flange size)

2.2 Construction

• Cast aluminium alloy housing, hermetically sealed against moisture and suitable for outdoor service.
• Upper float (alarm): tilts as gas accumulates, actuating one mercury-tilt switch for alarm / signal output.
• Lower float (trip): responds to rapid oil flow / oil-level drop, actuating a second mercury-tilt switch for tripping.
• Two independent mercury-tilt switches (SPDT), one per float, for reliable alarm and trip contacts.
• Transparent inspection window for visual check of oil level, float position and gas accumulation.
• Top vent / test valve for manual gas release and functional testing without dismantling the relay.

3. Technical Data

Parameter	Specification
Model	QJ12-25
Connection diameter	DN25
Protection principle	Dual float (upper = alarm, lower = trip), no oil-flow baffle
Gas volume for alarm	200 – 250 cm³ (adjustable, factory set)
Trip actuation	Lower float displacement by rapid oil flow / gas surge
Contact type	2 x SPDT mercury-tilt switch (alarm + trip)
Rated voltage / current	AC 220 V / DC 220 V, 5 A (resistive load, to be confirmed)
Operating temperature	-40 °C to +85 °C
Housing material	Cast aluminium alloy, outdoor grade
Ingress protection	IP65 (terminals), sealed body
Mounting orientation	Horizontal, in tank-to-conservator pipe, with slight slope toward conservator
Compliance	IEC 60076-1, GB/T 6451

4. Key Features

• Dual-float, two-level protection: independent upper and lower floats clearly separate alarm from trip functions, avoiding false tripping.
• Mercury-tilt switch output: sealed, self-cleaning contacts with stable ON/OFF signalling for both stages.
• No oil-flow baffle: simpler internal structure and faster response to internal faults compared with baffle-type relays.
• Clear visual indication: transparent window allows at-a-glance inspection of oil level, float status and gas build-up.
• Outdoor reliability: hermetically sealed cast housing resists humidity, vibration and harsh site conditions.
• Easy field testing: vent / test valve permits gas release and functional verification without removal.

5. Applications

• Small to medium power transformers (distribution and pole-mounted units) using DN25 conservator piping.
• Oil-immersed reactors, transformers and tap changers requiring compact two-stage protection.
• Retrofit and upgrade projects replacing older single-float or baffle-type relays.
• Substations and industrial sites seeking reliable incipient-fault detection with alarm + trip separation.

6. Why Choose Us

• Customisation: connection flange, float setting and contact configuration tailored to your transformer design.
• 100% factory calibration: every relay is tested for float action, contact operation and seal integrity before shipment.
• Proven reliability: robust cast housing and mercury switches ensure long service life under outdoor conditions.
• Fast response: engineering support and quotation within 24 hours.

7. Ordering Information

Specify when ordering	Example / Options
Transformer rated power & voltage	e.g. 10 MVA, 33/11 kV
Connection / flange type	DN25, flange standard (e.g. GB / DIN / ANSI)
Contact / output requirement	2 x SPDT (alarm + trip), mercury switch
Set values	Alarm gas volume 200-250 cm³; trip by lower float (confirm with design)
Quantity & destination port	As required
Request a Free QuotationSend us your transformer / piping drawing (DWG / DXF / PDF) and we will return a complete design proposal within 24 hours.