Delta Controls 2HT - Manual (Page 7)

002522-2HT 180210.doc 7
MECHANICAL INSTALLATION
Conduit connection
The 2HT series Transmitters are supplied with either M20 x 1.5 or 1/2" NPT internally threaded electrical
connection. To prevent condensation build-up in the conduit entering the instrument housing, the
following piping layout should be followed. Additional considerations must be made if transmitters are to
be mounted in hazardous areas.
If cable is used a suitable gland should be chosen so that protection rating and hazardous area
certification is not affected, in accordance with site or national codes of practice.
ELECTRICAL INSTALLATION
Cable selection
The HART protocol is designed to be used over conventional instrument cables, but there are limitations
on the usable length, depending on the type of cable used. These limitation are concerned with two
things :-
(i) keeping interference at a level low enough not to affect the reliability of the communication, and
(ii) ensuring a proper signal level at the receiver.
Interference.
Each HART loop consists of a pair of wires, these should be twisted together to reduce interference
from external electromagnetic fields. Please note that the full benefit of a twisted pair is not obtained
because the HART signal is not symmetrically balanced on the two wires. For all but the very shortest
cable runs, screening is necessary for protection against interference. There may be a single screen
covering a number of twisted cables, or, better, each twisted pair may have its own individual screen. All
screens should be connected together and connected to ground at one point only, typically near the
control room.
Signal Level
The main factor that affects the signal level is cable capacitance. The signal level is degraded as the
cable capacitance rises. As a general rule the product of the total resistance and the total capacitance
must be less than 65 microseconds.
( Rload + Rbarrier + Rmiscellaneous ) ohms x ( Ccable + Cdevices ) farads <= 65uS
Cable capacitance is measured per meter so multiply the nominal value by the length of cable run. For
each HART device allow 5 nF x the CN number.
To limit the possible effects of interference and signal degradation the following rules are imposed in the
HART specification :-
Maximum overall length, individually screened pairs :- 3000 metres
Maximum length, multiple pairs with only an overall screen :-
1500 metres