This is a great question, and it highlights a common point of confusion when dealing with separately derived systems and grounding. Let's break it down

Key Concepts:
Separately Derived System (SDS)

This is a power system where the neutral is connected to ground at a single point within the system (often at the transformer or generator). The key is that it's electrically isolated from the other power systems upstream of its source. Emergency generators, UPS systems, and lighting inverters typically supply SDS.

Equipment Grounding Conductor (EGC)

The EGC is the path that provides a low-impedance fault current path back to the source, allowing the overcurrent protection device (breaker or fuse) to trip in the event of a ground fault. It's crucial for safety.

Bonding

Bonding is the intentional electrical connection of all metal enclosures, raceways, and other non-current-carrying metal parts to create a low-impedance path to facilitate the operation of overcurrent devices.

The Scenario: Mixed Emergency and Normal Power in the Same Room

You have emergency lights and normal lights in the same room, with a relay controlling them. Both systems' equipment grounding conductors are tied together in a "mixing box."What Happens During a Ground Fault?

This is where it gets interesting. Here's what would likely happen in the event of a ground fault on the emergency power side:1. Fault Current Path

The fault current will flow from the point of the fault, through the EGC system (which now includes the mixed EGCs in the mixing box), and back to the source of the emergency power. This is where the grounding and bonding come into play.2.

Primarily Back to the EM Panel

The vast majority of the fault current will flow back to the emergency panel. The reason is that this is the lowest impedance path. Remember, electricity does take all paths, but it follows the path of least resistance (impedance, actually). The emergency panel is the closest source, and its grounding electrode system (GES) establishes its connection to ground.3.

Some Current May Flow to the Normal Panel (but a small amount)

Yes, some small portion of the fault current could theoretically flow from the mixing box, back through the normal system's EGCs, and toward the normal panel's ground point. However, the impedance of this path is much higher for a few reasons:

Distance

The normal panel is further away, adding length and therefore impedance to the conductor run.

Bonding Impedance

Any connections in the grounding system (e.g., connectors, terminations) will have some small amount of impedance. The cumulative effect of these impedances makes the path back to the normal panel less attractive to the fault current.

System Grounding Electrode System

Both SDS should have their own grounding electrode systems (GES). The normal panel's GES creates a local ground reference.

Why Tying the EGCs Together is Generally Acceptable (and often Required)



Equipotential Bonding

By connecting the EGCs together, you create what is known as an equipotential plane. This means that during a fault, all metallic parts within the accessible space will be at nearly the same potential. This is critically important for preventing dangerous voltage differences between metallic objects that a person could touch simultaneously.

Safety

Even if a small amount of current flows through the normal system's grounding, it helps trip the EM breaker faster, which is the goal.

Important Considerations and Best Practices



NEC Compliance

Always follow the National Electrical Code (NEC) and local codes. The NEC outlines specific rules for grounding and bonding separately derived systems.

Proper Grounding Electrode Systems

Ensure that both the normal and emergency systems have properly installed and maintained grounding electrode systems (GES). This is crucial for effective fault clearing.

Wire Size

Use appropriate EGC sizes as specified in the NEC.

Good Connections

Ensure all grounding and bonding connections are clean, tight, and corrosion-resistant. Poor connections increase impedance and can hinder the fault clearing process.

Qualified Personnel

Only qualified and experienced electricians should perform work on electrical systems, especially when dealing with separately derived systems.

In Summary:

When you tie the equipment grounding conductors of separately derived systems together in a mixing box, you're creating a common grounding path. In the event of a ground fault, the majority of the fault current will flow back to the source of the power (in your example, the emergency panel) because it's the path of least impedance. However, a small amount of current could* theoretically flow through the other system's grounding, but this should be negligible. Tying the EGCs together is generally safe and often required to maintain equipotential bonding and enhance safety.I hope this explanation helps clarify things! Let me know if you have any other questions. Flag for review