You're right, it's unusual to see two separate circuits feeding a single light fixture (EM and normal). Here's a breakdown of why it might be done, and the most common reasons

The Most Likely Reason: Emergency Lighting
Emergency Lighting (EM Circuit)

The most common reason for a second circuit is to provide emergency lighting. This circuit is connected to an emergency power source (like a generator or battery backup) and is designed to keep the light on during a power outage. This is a critical safety feature in many buildings, especially commercial or public spaces.

Why Two Circuits for Emergency Lighting? Compliance Codes

Building codes often mandate emergency lighting in specific areas (exit routes, stairwells, etc.). These lights must remain illuminated even if the normal power fails.

Testing Functionality

Having a dedicated EM circuit allows for periodic testing of the emergency lighting system without disrupting the normal lighting circuit.

Separation of Power

For safety, it is common to have the emergency power and normal power completely isolated until necessary. This reduces the chance of faults in the normal lighting circuit impacting the emergency lighting.

How it Typically Works

The "normal" circuit powers the light under standard conditions. If the normal power fails, a transfer switch (often built into the light fixture itself, or a separate device in the circuit) automatically switches the light over to the EM circuit. When normal power is restored, the light switches back to the normal circuit, and the EM circuit might recharge its batteries if it's a battery-backed system.

Other Potential (Less Common) Reasons



Redundancy

In critical environments (like hospitals or data centers), a second circuit might be for redundancy. If the first circuit fails, the second one can take over to keep the light operational.

Multiple Lighting Levels

Occasionally, two circuits might be used to control different parts of the light fixture for varying light levels (e.g., one circuit for dim ambient lighting, another for bright task lighting).

Specific Control Requirements

Sometimes, a specific building automation system (BAS) or lighting control system might require a separate circuit for a particular light to allow for very precise control or monitoring.

Old Wiring Practices

In very old installations, you might encounter unusual wiring configurations that don't conform to current standards. However, this is less likely.

Important Considerations



Safety

Electrical work is dangerous. If you're not a qualified electrician,

do not attempt to work on these circuits yourself. Labeling

All circuits and switches should be clearly labeled to indicate which circuit is normal and which is emergency.

Testing

Emergency lighting systems should be regularly tested to ensure they are functioning correctly.

In summary, the presence of two circuits (EM and normal) is almost certainly due to emergency lighting requirements. It's a critical safety feature that ensures the light will function during a power outage.To be absolutely sure, you'd need to trace the circuits back to their source and identify what type of power supply is feeding each circuit. If one leads to a battery backup, generator, or emergency power panel, then you've found your answer. Flag for review