Long Time Delay
Long Time Delay (LTD) is the intentional, adjustable delay in tripping of a circuit breaker or protective relay at moderate overcurrent levels, allowing normal inrush or temporary overloads while protecting conductors and equipment from sustained thermal damage. It is part of time-current characteristics in molded-case and electronic trip breakers, providing inverse time tripping over seconds to minutes.
On the shop floor, Long Time Delay is applied in power distribution protection, motor control centers, and ground-fault coordination. Main and feeder breakers have adjustable long-time pickup and delay settings to ensure selective coordination, where downstream breakers trip first for local faults, and conductors are protected from prolonged overloads. In motor control centers, long-time characteristics protect motors and cables from overload without tripping during start-up inrush. Proper settings balance thermal limits, arc-flash mitigation, and equipment uptime, directly impacting fault clearing time, arc-flash incident energy, and shock risk to workers.
- Mis-set long-time delay (too long) increases arc-flash and fire risk, often due to lack of documented coordination or arc-flash studies, or ad-hoc changes without engineering review.
- Disabled or bypassed time-delay protection, such as switching to instantaneous only or increasing settings to avoid trips, leads to conductors operating above design temperature and motors running in overload, violating manufacturer instructions and OSHA requirements.
- No integration of long-time delay behavior into lockout or task-specific risk assessments, where workers assume instant trip on fault, ignoring time-current curves, leading to misjudged fault durations and inadequate emergency response planning.
How is Long Time Delay different from Long Periods of Delay in WorkSafeBC OHS Regulation?
Long Time Delay is an engineering term for the time-current characteristic of protective devices that delays trip at moderate overcurrent levels. WorkSafeBC's former 'Long periods of delay' (section 18.16) dealt with delays in emergency response reporting and was repealed in 2021. For SafeDesk, any reference to 18.16 is historical; current practice focuses on protective device behavior and general hazard-control duties.
What technical parameters of Long Time Delay should a facility document or control?
Key parameters include long-time pickup setting (e.g., 0.8–1.0 × In), long-time delay setting (seconds at a given current multiple), manufacturer-specified time-current curves, thermal limits of conductors and equipment, and arc-flash studies that incorporate actual clearing times. SafeDesk should prompt for current protective device coordination and arc-flash studies, and verify settings against them.
How does Long Time Delay interact with WorkSafeBC expectations about correcting hazardous conditions without delay?
WorkSafeBC requires unsafe conditions found during inspections to be remedied without delay. If long-time delay settings cause overheated conductors or elevated arc-flash risk, these are unsafe conditions that must be corrected promptly by adjusting settings or updating studies. Long Time Delay itself is necessary for coordination, but compliance issues arise when it is not engineered or results in known hazards.