Okay, let's break down the issue of modeling a relay in SKM (or any power systems modeling software) and how it relates to your description. Your understanding is partially correct, but the critical point is how the relay functions to control the circuit.

Understanding Relays in Power Systems

Relays are control devices, not just passive components. Their primary job is to detect a condition (overcurrent, undervoltage, ground fault, etc.) and then initiate a change in the circuit, typically by opening a circuit breaker or switch. They don't directly carry the main circuit current (at least, not the way a fuse or switch does). The relay controls something that does carry the main current.

Key Considerations for Modeling Relays in SKM


Relay as a Control Element

You need to represent the relay's protective function. This means:

Setting

You must input the relay's settings (pickup current, time delay, etc.) based on the desired protection scheme. This is crucial because SKM needs to know when the relay should trip.

Coordination

You need to coordinate the relay's settings with other protective devices (fuses, other relays) to ensure proper selectivity (the right device trips first).

Trip Output

Crucially, you need to tell SKM what the relay controls when it trips. This is usually a circuit breaker, but could be a switch.2.

The Controlled Device (Switch or Breaker)

This is the element that interrupts the current. The relay tells it to do so.3.

Physical Location vs. Electrical Function

While the relay's physical location in the switchboard is helpful for understanding, the electrical function is what matters most for modeling.

Analyzing Your SKM Snapshot (Based on Your Description) Incoming Line -> Switch -> Relay -> Fuse -> Switchboard Bus

This physical arrangement is common. The relay monitors the current/voltage after the switch, and before the fuse. But it doesn't directly interrupt that current path.

Problem

If the relay is simply placed in the current path like a passive device, SKM won't recognize it as a control device. It won't simulate the relay's protection function (detecting a fault and initiating a trip). The fuse will see the current coming through after the relay, and the relay will simply be just another component with an impedance and voltage drop.

Correct Modeling Approach

Here's how you should model it in SKM (general steps - specifics may vary depending on the SKM version):1. Model the Switchboard Bus

This is your downstream load point.2.

Model the Fuse

Place the fuse directly upstream of the bus. Define its fuse type and characteristics.3.

Model the Switch

Place the switch directly upstream of the fuse.4.

Model the Incoming Line

Connect the line to the switch.5.

Crucially, Model the Relay's Control Function

This is the tricky part. Instead of just sticking the relay in the current path, you need to tell SKM that the relay controls the switch. This is usually done in one of the following ways (depending on the SKM version):

Protective Device Coordination Module

SKM (and other software) usually has a module specifically for protective device coordination. This is the ideal place to model the relay. You would: Define the relay type (e.g., overcurrent, undervoltage, etc.). Enter the relay's settings (pickup current, time delay curves, etc.).

Associate the relay with the switch. This is the key step. You tell SKM that when the relay trips, it opens the switch. Relay as a Trip Device for the Switch

Some SKM versions might allow you to define the switch as having an external "trip device." You would then model the relay as that trip device, specifying its settings.

In Summary



Don't just put the relay "in line." Focus on the relay's control function. Use SKM's protective device coordination tools to define the relay settings and, most importantly, link* the relay to the switch (or circuit breaker) that it controls.Example (Conceptual SKM Steps):1. Open SKM.
2. Create your one-line diagram: Line -> Switch -> Fuse -> Bus.3. Right-click on the switch. Choose "Protective Device Coordination" or a similar option.
4. In the coordination module, add a new relay.5. Select the relay type (e.g., "51 Overcurrent Relay").
6. Enter the relay's settings (e.g., 5A pickup, inverse time curve).7. In the relay settings, there should be a field to "Control" or "Trip" a device. Select the switch from the list. This tells SKM that when the relay trips, it will open the switch.
8. Run a coordination study to verify that the relay trips before the fuse under fault conditions.By following these steps, you'll be accurately modeling the relay's protection function in SKM, not just its physical location. If you can provide the SKM version you are using, I can provide more specific details on how to model the relay's control function. Flag for review