The MTL TP48 series of surge protection devices uniquely provide a level of protection for 2, 3 and 4 wire field-mounted transmitters that is far in excess of the optional transient protection facilities available from the transmitter manufacturers without involving any additional wiring, conduit modifications or other expensive extras.
The TP48 range protection network consists of high-power, solid-state electronics and a gas-filled discharge tube capable of diverting 20kA impulses. The whole unit is encased in a 316 stainless steel housing, threaded for the common conduit entries used on process transmitters. Versions are available for 1/2" NPT, 20mm ISO, and G1/2" (BSP 1/2 inch) threaded entries.
Installation can easily be carried out retrospectively to existing installations. The TP48 is screwed into any unused conduit entry on the transmitter case and flying leads are connected to the terminal block (+ve, -ve) and the internal earth stud. The 3 wire TP protects +ve, -ve and signal. The 4 wire TP48 protects +ve, -ve, signal +ve and -ve. The TPs operate without in any way affecting normal operation - passing ac or dc signals without attenuation while diverting surge currents safely to earth and clamping output voltages to specific levels.
The all-important earthing connection is made to the local casing of the transmitter with no separate earth connection or ground stake at the transmitter being needed. In operation, the TP48 makes sure that the transmitter electronics are never exposed to damaging transients between lines or between lines and casing/earth. Any surge current appearing as a series-mode or common-mode transient is converted into a common-mode voltage - whereupon the transmitter electronics are temporarily raised to some higher voltage level before ‘floating' down automatically (and without damage) to resume normal operation.
For hazardous area use, approvals for both intrinsically safe and flameproof (explosion-proof) operation are available (pending for the TP48 3 & 4 wire), in all gas groups and apparatus temperature classification up to T4 for the TP48 3 & 4 wire and T6 for the TP48. Where transmitters are used in circuits suitable for Div 2/Zone 2 installations, the TP can be added without adversely affecting the level of safety.
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