The MTL TP24/7 series of surge protection devices is a unique unit providing a level of protection for 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 TP24/7 protection network consists of high-power, solid-state electronics and a gas-filled discharge tube capable of diverting 10kA 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 is simple and can easily be carried out retrospectively to existing installations. The TP24/7 is screwed into any unused conduit entry on the transmitter case and flying leads are connected to the terminal block (+ve, -ve), RS485 terminals and the internal earth stud. They 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 TP24/7 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, in all gas groups and apparatus temperature classification up to T4. Where transmitters are used in circuits suitable for Div 2/Zone 2 installations, the TP24/7 can be added without adversely affecting the level of safety.
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