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Differences Between Wet and Dry Alarm Contacts on RTUs

By Andrew Erickson

July 2, 2024

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One of our clients recently sent us a web chat about integrating a NetGuardian RTU with their remote monitoring system. The primary goal was to ensure solid communication between two critical pieces of equipment. This conversation highlights the detailed steps and considerations involved in achieving this integration.

In particular, this discussion revolved around the wet vs. dry contact closures that RTUs expect from connected equipment. This client wanted to be sure that the NetGuardian would match with their monitored equipment once deployed.

As we analyze this chat, we will learn the steps needed for this integration. We will also see real examples of the technical expertise and support provided by DPS Telecom. Let's dive in:

Full Client Conversation Transcript (Web Chat)

Client: What is the minimum current requirement for alarm dry contact input to NetGuardian?

Andrew: It's pretty small. We provide it on the ALM (alarm) pin for each discrete input. Are you needing the figure because you plan to provide a wet contact point?

Client: What is the minimum alarm input current/max voltage for NetGuardian? I need the amount in milliamps to connect to an outside controller without the need for interposing relays. The controller output is rated at 1/2 Amp at 24V AC (500 mA).

Andrew: That will be plenty. The RTN (return) pin is essentially just a continuity tester. The voltage is a consideration, whether TTL circuit or not.

Client: It does not specify whether TTL or other. It would be the Output dry contact spec in the UC600 spec snippet.

Andrew: Hmmm, let me review this with Engineering down the hall to be sure. Okay, here's what I got: Yes, we can probably work with wet Form-A. We'd want our RTU to be a 24V-powered unit & not TTL. That makes the 24V compatible. Your relay provides 24V in one position. In the other, will it be floating? Connected to Ground?

Client: The contact is a dry contact from the UC600, the NetGuardian is -48VDC. (Somehow the Edge browser closed on me and I had to re-establish. Luckily I see your message.)

Andrew: I only see wet contact outputs from the UC600 you sent. It has dry contact inputs, so to connect it to the NetGuardian for monitoring, we'd need to handle wet relay, not dry. Almost all NetGuardians can be built as 24VDC or 48VDC, so don't worry about that part. I have retrieved the full manual for UC600.

Client: See the pic. They have a separate line for the relay 4 outputs.

Andrew: Yes, I follow. If the goal is to monitor the UC600 from a NetGuardian, that would mean that those wet relay outputs must be connected to our discrete inputs. We normally expect a dry contact relay, but we can work with wet, as noted above.

Client: These are dry relay contacts rated at 500mA at 24 VAC. We need to use them as alarm/status inputs into the NetGuardian. At the NetGuardian, these dry contacts shall be wetted with the other alarms, 48V system. We already have the NetGuardian installed.

Andrew: I see, thank you for that detail about the NetGuardian already being installed. Let me see what I can do for you there about making these two play well together.

Client: I think they were 5mA input required at the NetGuardian inputs?

Andrew: Running this by a hardware engineer now. Okay, I'm back. Apologies for not noticing VAC right away. AC presents a fundamental problem for our discrete inputs that expect to see DC flowing into the RTN pin. You'll need an external coil that latches a dry contact when AC is present.

Client: These are dry contacts, but the relay is AC. We just need to know what the NetGuardian inputs require or the amount of current the NetGuardian inputs will draw when activated. The NetGuardian inputs are DC. An AC rated contact will also work in DC systems, except, in this case the 500mA gets derated.

Andrew: The NetGuardian inputs are not DC inputs; they are weakly biased inputs looking for closure to Ground. Your external dry contacts are perfect for our discrete inputs.

Client: Correct. We are looking to establish the make or break current as determined by your weakly biased inputs.

Andrew: The current that flows through the circuit when a NetGuardian discrete is closed to Ground is a nominal (<4mA) current.

Client: Great! The UC600 outputs at 500mA should easily accommodate the NetGuardian input requirements.

Andrew: I agree. If the UC600 outputs are rated at 500mA, they should be more than sufficient for the NetGuardian's discrete inputs. Apologies for my confusion.

Client: Thanks.

Understanding the Client's Wetted-Relay Requirements

The client initiated the conversation. They opened by inquiring about the minimum current requirements for the alarm dry contact input to the NetGuardian:

"What is the minimum current requirement for alarm dry contact input to NetGuardian?"

The NetGuardian RTUs are designed to be highly adaptable. The device supports various configurations to meet specific needs.

For this client, their output was rated at 1/2 Amp at 24V AC. They needed to understand how to interface this with the NetGuardian's discrete inputs without using interposing relays.

500mA was More Than Enough Current Capacity

Andrew from DPS Telecom provided an initial response and asked follow-up questions. He indicated that the NetGuardian's alarm pin could handle a small current.

This makes it suitable for most dry contact applications. However, the client needed specific details to ensure compatibility with their 24V AC outputs.

Andrew inquired further:

"It's pretty small. We provide it on the ALM (alarm) pin for each discrete input. Are you needing the figure because you plan to provide a wet contact point?"

The chat points to the need for precise technical specifications. In this case, it made sure the NetGuardian RTU could interface correctly with the UC600's outputs.

The client's controller was capable of handling 500 mA. This posed a compatibility question for the NetGuardian's discrete inputs. These inputs are typically designed for DC circuits.

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Addressing Voltage Compatibility Challenges

During the integration process, the client faced voltage challenges. The client was looking to guarantee the voltage specifications of the UC600 outputs aligned with the NetGuardian's requirements. Specifically, the client wanted the 24V AC outputs from the UC600 to be managed without additional devices or technologies.

Andrew provided clarity on this issue by explaining the voltage compatibility requirements of the NetGuardian. He confirmed that the NetGuardian could handle the 24V outputs from the UC600 if the correct configuration was in place:

"We normally expect a dry contact relay, but we can work with wet, as noted above."

This insight was crucial in determining that the NetGuardian could indeed handle the 24V outputs from the UC600. This compatibility enables seamless monitoring and control.

Understanding How Discrete Inputs Work is Crucial for Good Alarm Monitoring

A significant part of the integration involved making sure that the alarm monitoring capabilities of the NetGuardian were fully utilized. The client needed to make sure that the discrete inputs on the NetGuardian could reliably detect and report any status changes from the UC600.

Andrew reassured the client by detailing the current requirements for activating the NetGuardian inputs:

"The current that flows through the circuit when a NetGuardian discrete is closed to Ground is a nominal (<4mA) current."

This information confirmed that the UC600 outputs could consistently trigger the NetGuardian inputs. This means the client can receive accurate and timely alarm notifications. The timeliness of these responses are critical for maintaining the operational efficiency and safety of the HVAC system.

Resolving the AC vs. DC Compatibility Issue

One significant challenge was the difference between AC outputs from the UC600 and the DC inputs expected by the NetGuardian. Andrew provided the client with a clear solution:

"AC presents a fundamental problem for our discrete inputs that expect to see DC flowing into the RTN pin. You'll need an external coil that latches a dry contact when AC is present."

This solution involved using an external coil to convert the AC signals to a dry contact. This makes them compatible with the NetGuardian's inputs. This adjustment ensured reliable operation and accurate alarm monitoring.

DPS Team

Leveraging Engineering Expertise

The chat shows the importance of having access to experienced engineers who can provide detailed, technical support. Having the Engineering Department at our headquarters in Fresno, CA isn't the cheapest choice to make, but it has amazing benefits for your tech support conversations.

Throughout the web chat, Andrew frequently consulted with hardware engineers to provide precise and accurate solutions to the client's questions:

"Running this by a hardware engineer now. Okay, I'm back."

The ability to consult with engineers in real-time ensured that the solutions provided were clear and correct. This allowed for a smooth and successful integration process.

Finding the Solution for You

Through expert consultation and tailored solutions, DPS Telecom was able to assist the client in achieving their goals. This included a seamless integration of their devices. DPS is always happy to help enhance your monitoring and control capabilities.

If you have any questions about maximizing your operations or achieving a seamless integration of your systems, reach out to our team of experts today!

Call 1-800-693-0351 or email sales@dpstele.com.

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Andrew Erickson

Andrew Erickson

Andrew Erickson is an Application Engineer at DPS Telecom, a manufacturer of semi-custom remote alarm monitoring systems based in Fresno, California. Andrew brings more than 17 years of experience building site monitoring solutions, developing intuitive user interfaces and documentation, and opt...