Under Voltage Protection
Does OSHA explicitly require under-voltage protection on metalworking or manufacturing machines?
OSHA does not have a standard written specifically for metal sawing machines, but under 1910.303(b)(1), equipment must be free from recognized hazards. OSHA identifies automatic restarting after power failure as a recognized hazard and notes that properly designed and maintained undervoltage equipment that prevents restarting is a suitable hazard control.
How does UVP differ from no-voltage release and shunt trip in industrial controls?
UVP (ANSI 27) continuously monitors voltage and trips when voltage falls below a configurable threshold for a defined time. No-voltage release coils are mechanical/electromagnetic implementations in contactors that hold the contactor closed only while control voltage is present, achieving prevention of automatic restart. Shunt trip is a separate coil that, when energized, immediately trips a breaker or contactor, often driven by a UV relay.
What typical settings and response times are used for UVP in plant power and ATS applications?
Typical UVP settings include setpoint ranges of 40-100% of nominal voltage, with common values around 95-97% for alarms/trips. Timer ranges are 0.1-100 seconds, with typical settings of several seconds to avoid nuisance trips. Relay operate time is <100 ms. For motors, thresholds of 85-90% are common, while ATS schemes use higher thresholds and delays of 5-10 seconds.
Under Voltage Protection (UVP) is a control and protective function that monitors supply voltage and, when the voltage drops below a preset threshold, trips or de-energizes equipment and prevents automatic restart when power returns, unless a deliberate restart action is taken by the operator. It is typically implemented via ANSI device 27 relays or undervoltage release coils in contactors.
On the shop floor, UVP is applied to machine main contactors, motor starters, and control circuits to ensure that equipment de-energizes during voltage drops or outages and does not automatically restart when power is restored. This prevents unexpected machine movement, which is a recognized hazard under OSHA 1910.303(b)(1). UVP is also used in automatic transfer switches to trigger backup power when utility voltage falls below acceptable levels. Typical settings include trip thresholds of 85-90% of nominal voltage with delays of 2-10 seconds to avoid nuisance trips. Proper UVP implementation is critical for compliance with electrical safety standards and safe work procedures.
Defeated or bypassed undervoltage protection leading to automatic restart, which OSHA identifies as a recognized hazard and can be cited under 1910.303(b)(1).
Incorrect UVP settings (e.g., setpoint too low or delay too long) causing equipment to run at damaging low voltages, or settings too high causing nuisance trips that prompt operators to disable protection.
Poor integration with lockout/tagout procedures, where UVP is mistakenly treated as a substitute for proper LOTO, leading to exposure to energized equipment and compliance violations.