Electrical Interlock for LV/HT Switchgear: Ensuring Safe and Reliable Operation
Introduction
Electrical interlocks in low voltage (LV) and high tension (HT) switchgear are essential for ensuring that switching operations are performed safely, avoiding equipment damage and preventing unsafe electrical conditions. Whether energizing a downstream switchboard through a transformer or interconnecting feeders, proper interlock schemes are critical to controlling sequence, preventing parallel supply hazards, and maintaining system stability.
This article explains:
- The role of electrical interlocks in LV/HT switchgear
- The “two-out-of-three paralleling” scheme
- Synchronism checking methods
- Common applications in industrial power systems
1. What Is an Electrical Interlock in Switchgear?
An electrical interlock is a control logic arrangement, often using auxiliary switches, relays, and control wiring, that prevents unsafe or incorrect switching sequences.
Example:
When energizing a downstream switchboard:
- Upstream switching device (breaker) must close first.
- Downstream breaker is closed next.
- If either breaker trips due to a fault, auxiliary circuits can trigger the other to trip as well — preventing backfeeding and fault propagation.
This approach ensures that power flow is controlled and system stability is maintained.
2. The “Two-Out-of-Three Paralleling” Scheme
This is a specialized closing sequence used when a switchboard has two parallel feeders (typically transformers) and a busbar section circuit breaker.
Objective:
- Allow no-break transfer between feeders
- Minimize duration of high fault levels at busbars during transfer
How It Works:
- Each of the three circuit breakers (two incomers + bus section) has auxiliary switches to detect closed status.
- If all three are closed, fault level may exceed safe limits.
- As soon as the third breaker closes, a signal is sent to trip one breaker (pre-selected via a selector switch).
- Some installations use a timer relay (0.5–2.0 seconds delay) before tripping to allow smooth load transfer.
Key Benefit: Prevents prolonged parallel operation, keeping fault levels manageable.
3. Synchronism Check Before Paralleling
When two supplies can be connected in parallel, it’s vital to ensure they are in synchronism and from the same source.
Method 1: Auxiliary Switches
- Installed on upstream circuit breakers (often busbar section breakers).
- Sends “breaker open” signals to prevent closure when an unsynchronised source is present.
- Inhibits closing of the third breaker in a two-out-of-three scheme.
Method 2: Synchronising Check Relay (ANSI Device 25)
- Monitors voltage, frequency, and phase angle across both sides of the breaker.
- Allows closure only if both sides match within preset limits.
- Installed on all three incomer breakers in a dual incomer switchboard.
4. Industrial Applications
Electrical interlocks are common in:
- Petrochemical plants – where high reliability and safety are mandatory
- Heavy industries – to maintain continuity during feeder transfers
- Power generation – for generator-to-grid synchronisation
- Critical facilities – such as data centers requiring no-break switching
5. Best Practices for Electrical Interlock Design
- Use redundant protection (both auxiliary switches and synchronising relays) for high-reliability systems
- Test interlock logic periodically to avoid malfunction during emergencies
- Clearly label interlock circuits and provide operating diagrams for operators
- Incorporate alarms for failed synchronism or abnormal paralleling attempts
Conclusion
Electrical interlocks in LV/HT switchgear are not just about preventing operator error , they protect equipment, improve reliability, and enable safe system transfers. Whether it’s the two-out-of-three paralleling scheme or synchronism checks, careful design and regular maintenance are essential for a safe and efficient power distribution network.