SF6 Density Relay Oil Leak: Causes & Solutions

1. Background

SF6 electrical equipment has been widely applied in power utilities and industrial enterprises, significantly advancing the development of the power industry. Ensuring the reliable and safe operation of SF6 equipment has become a critical task for power departments.

The arc-quenching and insulating medium in SF6 equipment is SF6 gas, which must remain sealed—any leakage compromises the reliability and safety of the equipment. Therefore, monitoring the SF6 gas density is essential.

Currently, mechanical pointer-type density relays are commonly used to monitor SF6 density. These relays provide functions such as alarm and lockout upon gas leakage, as well as on-site density indication. To improve shock resistance, these relays are typically filled with silicone oil.

However, in practice, it is common to encounter oil leakage from SF6 gas density relays. According to industry reports and feedback, this issue is widespread—every power supply bureau across China has experienced it. Some relays develop oil leaks within less than one year of operation. The problem affects all manufacturers, including both imported and domestic models. In short, oil leakage in oil-filled density relays is a prevalent and systemic issue.

2. Purpose of Filling with Silicone Oil

2.1 Improve Vibration Resistance
These density relays typically use a spiral spring (hairspring) type electrical contact. Although magnetic assistance enhances contact closure force, the actual contact pressure (for alarm or lockout signals) relies primarily on the weak force of the hairspring—even with magnetic assistance, it remains very small. As a result, the contacts are highly sensitive to vibration.

2.2 Protect Contacts from Oxidation
The relay uses magnetically assisted electrical contacts with inherently low contact pressure. Over time, oxidation can cause poor contact or complete signal failure. The silicone oil filling prevents air exposure, thereby protecting the contacts from oxidation and ensuring long-term reliability.

3. Hazards of Oil Leakage

Hazard 1: Loss of Damping and Reduced Shock Resistance
Once the anti-vibration oil fully leaks out, the damping effect is lost, drastically reducing the relay’s vibration resistance. Under strong mechanical shocks during circuit breaker opening/closing operations, the relay may suffer from:

• Pointer jamming

• Permanent contact failure (stuck open or closed)

• Excessive measurement deviation

Hazard 2: Contact Oxidation and Contamination
In oil-leaked relays, the magnetically assisted contacts are exposed to air, making them prone to oxidation and dust accumulation. This leads to unreliable contact or complete signal interruption. If the density relay fails due to a stuck pointer or faulty contacts, it cannot detect actual SF6 gas loss.

Imagine an SF6 circuit breaker losing its insulating gas, yet the density relay fails to trigger alarm or lockout due to internal failure—and then attempts to interrupt fault current. The consequences could be catastrophic.

Additionally, leaked oil contaminates other switch components, attracts dust, and further compromises the safe operation of the SF6 switchgear.

4. Root Cause Analysis of Oil Leakage

Oil leakage primarily occurs at three locations:

4.1 Internal Leakage in the 7-Pin Terminal Box
Signal outputs from the relay require electrical connections from inside to outside the case, using a 7-pin plastic connector. The internal pins are made of copper, while the housing is plastic. The assembly is produced via overmolding (casting). Due to the different thermal expansion coefficients of metal and plastic, temperature fluctuations can create micro-cracks or gaps at the interface, leading to oil leakage.

4.2 Leakage at the Joint Between the 7-Pin Box and Case
This joint is sealed with an O-ring gasket. Under normal conditions, leakage is rare. However, when internal pressure increases or large temperature differences occur between inside and outside the case, stress on the seal can cause oil to leak from this joint.

4.3 Leakage at the Dial Cover
Leakage here is less common and usually results from improper assembly by the manufacturer, such as inadequate sealing or misalignment during production.