At a Glance. Protecting a signal that enters a hazardous area means limiting the energy that can appear at the field terminals. There are two established ways to do it, and the choice changes the wiring, the earthing and often the cable specification. MTL7706+ is a single-channel shunt-diode barrier for 4–20 mA loops in Zone 0, and MTL7787+ is the two-channel version for digital signals with a 28 V / 93 Ω characteristic. A galvanic isolator achieves the same protection without relying on an intrinsically safe earth. All of these are catalogued under the MTL range.
1. How each technology establishes safety
Shunt-diode barriers
A shunt-diode barrier limits voltage with a chain of zener diodes and current with a resistor, and it relies on a high-integrity safety earth to divert the fault energy. If the earth connection is lost, the protection is lost. The barrier is passive — it needs no supply — and it is simple, robust and the lowest-cost option.
Galvanic isolators
A galvanic isolator separates the hazardous and safe sides optically or transformer-coupled. There is no reliance on a safety earth, so the installation rules are simpler and the isolator can be used where a suitable earth is not available or where a fully floating circuit is required. It needs a supply and costs more per channel.
2. Comparison
| Criterion | Shunt-diode barrier | Galvanic isolator |
|---|---|---|
| Safety earth required | Yes, high integrity, typically below 1 Ω | No |
| Auxiliary supply | None | Required |
| Cost per channel | Lower | Higher |
| Signal type | Analogue and digital versions | Wide range, including RTD and thermocouple |
| Earth-loop interference | Possible on long loops | Eliminated by isolation |
| HART transparency | Yes on appropriate models | Yes on appropriate models |
| Typical fit | Panel with a solid earth bar, cost-sensitive projects | New builds, retrofits without a safety earth, high channel counts |
3. Channel count and panel space
Shunt-diode barriers and isolators both come in single and multi-channel formats, and the density decision is usually made by the panel. MTL7706+ is a single-channel 4–20 mA barrier; MTL7787+ handles two digital channels in the same footprint, which halves the DIN-rail space for switch and proximity inputs. Where a large number of HART devices must reach the control system, a multiplexer such as MTL831B concentrates 32 channels onto a single Modbus or Ethernet link and is a far cheaper route than 32 individual isolators.
4. Don't forget the line conditioner
In plants with large drives and welders, induced noise on long 4–20 mA loops can corrupt HART communication even when the barrier is correctly installed. MTL4850 is a HART line conditioner that filters that induced noise while passing the loop current unchanged. It is a small addition that resolves a class of intermittent faults which are otherwise very hard to diagnose.
5. What to send for a quotation
- Number of channels, and whether the signal is analogue, digital or temperature
- Hazardous area classification: zone, gas group and temperature class
- Whether HART or fieldbus digital communication must pass through
- Whether a high-integrity safety earth is available at the panel, which is the deciding question between the two technologies
- Panel width available and the required terminal style
MTL has maintained backward compatibility across generations, but the suffix on the barrier designates approvals and channel configuration, not cosmetics. Send the full model and order number from the installed device label and we will confirm the current equivalent from the MTL stock with the datasheet attached to the quotation.