by Bob Bob

There are several different surge protectors that you can use to keep your electronics safe including transient protection options from the transmitter manufacturers themselves, but none compare to ZeroDT’s technology. We break down the difference between Rosemount’s offerings and the protection ZeroDT can provide.

The Difference Between ZeroDT and Rosemount

Rosemount is one of the major manufacturers in the remote electronic measurement of flow and pressure level. When you purchase one of their transmitters, you may have an internal option for transient protection that you can order. Or, they offer an external transient protection device called the Rosemount 470.

In comparison, ZeroDT is a relatively new technology that has been developed specifically to combat common problems that arise for users of surge protectors. ZeroDT offers several different devices which are appropriate for varying levels of voltage and can help with different numbers of wires and loops.

An Overview of the Rosemount Protectors

The Rosemount 470 Transient protector consists of a circuit that utilizes a Gas Discharge Tube, an inductor, and a Zener diode. The primary protection is provided by the GDT. The GDT will conduct large currents, but it also results in slow reaction time, degradation with usage, and a ‘glo-mode’ with DC currents.

To overcome this first deficiency – slow response time, Rosemount uses a small bipolar Zener diode. However, while the Zener diode is fast, it cannot handle large surge currents, so an inductor is used to try to limit the current through the Zener diode until the GDT can fire and take the surge current. The voltage that the transmitter electronics will see is the sum of the voltage across the Zener diode and the inductor until the GDT responds and starts conducting the surge current.

The second issue is that the electrodes that make the spark gaps in GDTs degrade every time that they activate and carry current. This leads to the turn-on voltage of the GDT becoming a higher and higher voltage as the electrodes degrade. This will mean that the Zener diode will have to conduct current for longer time periods and carry higher currents during the time period before the GDT activates. Because the Zener diode is not designed for these levels of current, or the duration of conduction and because of this, it fails and removes itself from the protection circuit without any indication to the user. The transmitter’s electronics are now protected solely by the GDT, which can have a slow response time and is degrading with continued usage until the transmitter is damaged or destroyed.

On top of this, when used in implementations where DC currents are involved, such as analog 4-20 mA loops, during a surge event the impedance of the GDT device drops and the voltage needed to maintain the arc drops well below the “spark-over voltage” for the device. The arc in the GDT will continue to conduct DC current and remain ‘lit’ even at 24 volts or lower. Because of this ‘glo-mode’ of the GDT, Rosemount has specified that unless the instrument power supply will limit the current to <0.5 Amps, fusing and additional impedance needs to be added to the circuit. This fusing can lead to fuse blows and downtime that require a tech to be dispatched to replace the fuse and bring the system back online.

Pros and Cons of the ZeroDT Protectors

The ZeroDT family of measurement surge protection devices do not rely on GDT, MOVs, or even Zener diodes. They ONLY utilize Silicon Avalanche Suppression Diode (SASD) technology. SASDs are designed to be able to handle the large surge currents that can be induced by lightning, while being as fast, or faster, than the Zener diodes.

Being a true semi-conductor material, they have a very low impedance to current flow once their turn-on voltage is reached and as long as the surge currents do not exceed the diode’s capacity, the SASD returns to its original state with no degradation in the device or its protection capabilities.

If the surge current exceeds the unit’s capabilities, the SASDs are designed to fail shorted to ground in a final attempt to protect the measurement device or transmitter. While it is true that SASDs may cost more than the other devices, the total cost of ownership can be much lower as they will continue to provide protection when other units may have been replaced, or the transmitters damaged not to mention the downtime that has occurred.

Because of this, the ZeroDT offers a very different and superior functionality to that of the Rosemount’s protective options.