The MTL TP-AC series of surge protection devices provides a level of protection for AC powered, field -mounted transmitters that is far in excess of the optional transient protection facilities available from the transmitter manufacturers without any additional wiring modifications or costly additions.
The TP-AC series protection network consists of high power, solid state electronics and a gas-filled discharge tube (GDT) capable of diverting 20kA impulses. The whole unit is encased in ANSI 316 stainless steel housing, threaded for attachment in conduit entries used on process transmitters. Versions are available with ½" NPT threaded entries.
Installation of the TP-AC can easily be carried out retrospectively to existing installations. The unit is screwed into any unused conduit entry on the transmitter case and flying leads are connected to the terminal block signal, power and earth terminals. The TP-AC operates – passing AC and 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 TP-AC 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 Fieldbus applications, the TP-AC Fieldbus models meet the requirement of IEC61158-2:2004 and ANSI/ ISA-50.02-2 1992 for 31.25kbit systems as used by Foundation Fieldbus™, Profibus PA and WorldFIP.
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