DS3800NRMA | Replacement for NRMA RTD Interface Board

Description

Product Introduction (Anti-Template)

A 3-wire RTD is only as accurate as its lead-wire compensation—if the termination board doesn’t handle the third wire correctly, you’re losing accuracy and you don’t even know it. The DS3800NRMA is GE’s dedicated RTD termination board for the Mark VIe system. It handles up to 16 RTD channels—2-, 3-, or 4-wire configurations—with precision current excitation and automatic lead-wire resistance cancellation. No external resistor networks, no manual scaling. Just wire the RTD and the board does the rest.

What sets the NRMA apart from a general-purpose analog board (like the NRCA) is the dedicated current source per channel: 0.5mA of precision excitation current that’s stable to 0.01% over temperature. That current drives the RTD, and the board measures the resulting voltage to calculate resistance—and from that, temperature. Compared to voltage-divider methods, the current-excitation approach gives you 10x better noise immunity and eliminates errors from lead-wire resistance. The NRMA also has per-channel lead-wire compensation built in, so a 3-wire RTD’s lead resistance (which can add several degrees of error) is automatically cancelled. A Midwest utility replaced their old 2-wire RTD terminations with NRMAs on 3-wire sensors and saw their bearing temperature readings stabilize from ±3°C to ±0.5°C—the lead-wire compensation made the difference.

Key Technical Specifications

Compatible Replacement Models

Frequently Asked Questions (FAQ)

What’s the difference between the NRMA and the NRCA?
The NRMA is a specialized RTD board with a precision current source (0.5mA) per channel, designed specifically for measuring resistance. The NRCA is a general-purpose analog board that can handle RTDs with external resistors, but it lacks the precision excitation and lead-wire compensation of the NRMA. The NRMA gives you 0.01°C resolution and ±0.1°C accuracy; the NRCA gives you about 0.1°C resolution and ±0.5°C accuracy. If you’re monitoring critical bearing temperatures, use the NRMA. If you’re just looking for a rough temperature reading, the NRCA is fine.

How does lead-wire compensation work on the NRMA?
For a 3-wire RTD, the NRMA measures the resistance of each lead wire and subtracts it from the total loop resistance to get the pure RTD resistance. This cancels the effect of lead resistance changes due to temperature, which can otherwise cause measurement errors of 0.1-0.5°C per ohm of lead resistance. The compensation is automatic—you just wire the RTD properly (the three wires go to specific terminals), and the board handles the math. The compensation works for lead resistances up to 50Ω, which covers about 1,000 feet of 20 AWG copper wire.

What is the excitation current on the NRMA and why does it matter?
The NRMA uses 0.5mA of excitation current per channel. That current flows through the RTD and generates a voltage that the board measures. The current source is stable to 0.01%, so the measurement is consistent. A higher current (like 1mA) would generate more voltage, but it would also self-heat the RTD, causing measurement errors. The 0.5mA current is the sweet spot—enough voltage for accurate measurement, but not enough to heat the RTD significantly. The self-heating error is about 0.01°C at 0.5mA, which is negligible.

Can I mix 2-, 3-, and 4-wire RTDs on the same NRMA board?
Yes, the NRMA supports all three wiring configurations. Each channel is independently configurable—you can have a 3-wire RTD on channel 1 and a 2-wire RTD on channel 2, etc. The configuration is done via jumpers on the board; refer to the manual for the specific jumper settings for each channel. If you use a 2-wire RTD, the lead-wire compensation isn’t active (you only have two wires), so you’ll have some lead-resistance error. We recommend 3- or 4-wire RTDs for best accuracy.

What is the difference between PT100 and PT1000 on the NRMA?
The NRMA supports both PT100 (100Ω at 0°C) and PT1000 (1000Ω at 0°C) RTDs. The excitation current is the same (0.5mA) for both, but the voltage drop is 10x higher for PT1000. The board automatically adjusts the gain based on the RTD type—you set the type in the I/O module configuration. PT1000 gives you better resolution and lower lead-wire error, but PT100 is the industry standard. Choose based on your sensor availability and accuracy requirements.

What’s the maximum distance I can run RTD wires from the NRMA?
The NRMA’s lead-wire compensation works for lead resistances up to 50Ω. For 20 AWG copper wire, that’s about 1,000 feet (300 meters) round-trip. For 22 AWG, it’s about 600 feet. If you have longer runs, you can use 4-wire RTDs, which eliminate lead-wire resistance entirely (the excitation current and sense wires are separate). For 4-wire RTDs, you can run up to 2,000 feet without significant error.

Can I hot-swap the NRMA while the rack is powered?
No. The NRMA is a passive board, but it’s connected to live RTD sensors and the I/O module. Pulling it while powered could damage the I/O module or create a voltage transient that affects other channels. Power down the rack, swap the board, and power back up. It’s a 15-minute job. Don’t hot-swap.

What do the LED indicators on the NRMA mean?

• Green: channel active and reading within range.
• Yellow: open circuit or over-range. The RTD is disconnected or the resistance is outside the measurement range (usually above 400Ω for PT100).
• Red: fault. The channel has an internal issue—could be a shorted RTD, a power supply problem, or a board fault.

If you see a yellow LED, check the RTD wiring—the sensor might be broken. If you see a red LED, power down the rack and inspect the board for visible damage.

What’s your test procedure for the NRMA?
Our inbound test:

• Visual: inspect for bent pins, cracked connectors, and check the resistor networks for damage.
• Excitation current test: verify each channel’s current source is 0.5mA ±0.01%.
• Lead-wire compensation test: simulate a 3-wire RTD with a known lead resistance (10Ω) and verify the board cancels the lead error.
• Accuracy: measure a known precision resistor (100Ω and 200Ω) and verify accuracy within ±0.1°C.
• Isolation: 500V DC between adjacent channels—must exceed 10MΩ.

We reject about 5% of inbound NRMA boards—most commonly for failed current sources or poor lead-wire compensation. The current source is the most sensitive component; it can be damaged by excessive voltage or current on the input.

What’s the typical lifespan of the NRMA?
In a climate-controlled environment, the NRMA has a typical lifespan of 15-20 years. The current sources are the most sensitive components—they can drift over time, but we’ve seen boards that are 12 years old still within spec. The terminal blocks and connectors are the mechanical weak points. The NRMA is a board you install and rarely think about—we recommend a calibration check every 5 years for critical RTD loops. The excitation current drifts slowly, but it’s worth verifying periodically.

Kollmorgen CB06551
Kollmorgen CB06551
EPRO PR9268/203-000
ABB 3BSE042235R2