OBC Detection, Lighting & Power common mistakes
OBC Detection, Lighting & Power · Understand the traps and how to avoid them.
Treating a sensor's rated detection distance as a guaranteed coverage radius and placing luminaires or field devices at that exact limit without allowing for beam pattern, mounting height, or environmental attenuation.
Datasheets advertise a single maximum range figure, so candidates anchor on that number and skip the coverage geometry, derating factors, and overlap requirements that real outboard installations demand.
What to do instead
Always convert the rated range into an effective coverage footprint using mounting height, beam angle, and a derating margin (commonly 30-50 percent for dust, vibration, or weather exposure), then verify overlap between adjacent sensor fields before finalizing layout.
Confusing the detection principle of a sensor (passive infrared versus active ultrasonic or radar) and selecting one for an application that its physics cannot serve, such as using a passive device to detect a stationary object.
Candidates memorize sensor names without linking each technology to the physical property it measures, so they misapply a motion-dependent sensor to a presence or ranging task.
What to do instead
Build a comparison table pairing each sensor type with the physical quantity it senses, its blind spots, and its failure conditions, then justify every selection in writing against those criteria.
Sizing a lighting circuit from the luminaire wattage alone while ignoring driver losses, inrush current, and the continuous-load 125 percent factor, leading to undersized conductors or nuisance trips.
Candidates treat the nameplate lamp rating as the total load and forget that LED drivers draw more than their output and that worst-case inrush can be many times steady-state current.
What to do instead
Compute branch load as luminaire input power plus driver loss, apply the continuous-load multiplier, and check breaker trip curve against measured or datasheet inrush before finalizing conductor and protection sizing.
Mixing luminaire types on a single dimmed or switched circuit without checking compatibility of dimming protocol, driver type, and minimum load.
Candidates assume any dimmable luminaire works with any dimmer, overlooking phase-cut versus low-voltage protocol mismatches and minimum-load thresholds that cause flicker or dropout.
What to do instead
Verify that every luminaire on a circuit shares the same control protocol and that total connected load exceeds the dimmer's stated minimum; document compatibility on the drawing before installation.
Selecting overcurrent protection by habit rather than by coordinating it with conductor ampacity, ambient temperature correction, and the equipment's short-circuit current rating.
Candidates default to a familiar breaker size and neglect that derated ampacity in a hot enclosure may be lower than the nominal conductor rating, or that the device's interrupting rating may be exceeded.
What to do instead
For each circuit, calculate corrected and adjusted conductor ampacity first, then choose protection at or below that value while confirming the available fault current is under the device's interrupting rating.
Ignoring voltage drop on long outboard runs and approving a conductor that satisfies ampacity but delivers unacceptable voltage at the load.
Candidates equate adequate ampacity with adequate design and forget that voltage drop scales with length and current independently of thermal capacity.
What to do instead
Perform a separate voltage-drop calculation on every long run and increase conductor size until the drop is within the project limit, documenting both ampacity and drop results.
Terminating stranded conductors in mechanical lugs without a listed connector or without torquing to the manufacturer's specification, leaving a high-resistance joint.
Candidates focus on getting the wire to stay in place and treat torque as optional, so strands flatten, oxidize, and loosen under thermal cycling.
What to do instead
Use connectors listed for the conductor class and material, torque every termination with a calibrated tool to the labeled value, and re-verify torque during inspection.
Treating grounding and bonding as the same function and omitting the bonding jumper between metallic enclosures and the grounding electrode conductor.
Candidates use the terms interchangeably, so they install a ground path to earth but leave metallic parts unconnected to it, defeating fault-current return.
What to do instead
Draw the grounding and bonding paths separately on every schematic, confirm each metallic enclosure has a low-impedance bond to the equipment grounding conductor, and test continuity end to end.
Performing a visual inspection and declaring the system sound without any live measurement, relying on appearance of terminations and luminaire illumination.
Candidates equate a working light or a tidy panel with a compliant installation and skip insulation resistance, continuity, and polarity tests that reveal latent faults.
What to do instead
Follow a written test sequence for every circuit: visual, continuity, insulation resistance, polarity, then functional load test, recording each result against the acceptance criterion.
Diagnosing a fault by replacing components in sequence instead of isolating the circuit into sections and measuring at defined test points.
Candidates lack a structured troubleshooting method, so they substitute parts and mistake a temporarily disturbed connection for a repaired root cause.
What to do instead
Adopt a divide-and-measure routine: split the circuit at accessible points, take readings against expected values, and confirm the root cause is corrected before restoring the protective device.