7828 Density Transmitter Technical Manual Electrical Installation
78285000_AB 3-3
Where permissible, the screen should be connected to earth at both ends. (At the 7828,
this is best done using a conductive cable gland.)
Cables should conform to BS2538. In the USA, use Belden 9402 (two-pair) or Beldon
85220 (single-pair). Other cables that are suitable are those that meet BS5308 Multipair
Instrumentation Types 1 and 2, Belden Types 9500, 9873, 9874, 9773, 9774 etc.
Maximum recommended cable length is 1000m (3200ft.), but care must be taken to
ensure that the power supply at the transmitter is at least 20V. Thus, for 24V power
supply, the overall resistance for the power supply connections (both wires in series) must
be less than 100 ohms.
In addition to the cable, to complete the wiring you will need:
•= ½” NPT to M20 gland adaptor
•= M20 x 1 cable gland
•= ½” NPT blanking plug
These are supplied with the 7828, but you may need alternatives to meet local electrical
installation regulations.
In hazardous areas these items must be explosion-proof.
3.2.5 SURGE PROTECTION
Careful consideration should be given to the likelihood of power supply surges or lightning
strikes. The power supply connections within the 7828 have a surge arrestor fitted to offer
protection against power supply transients and, to some extent, indirect lightning strikes.
If there is a possibility of lightning strikes, external surge protection devices - one for each
pair of signals and the power supply - should be installed as close to the 7828 as possible.
Another method is to connect an MOV (Metal Oxide Varistor) (breakdown voltage >30V)
with an NE-2 neon bulb in parallel across each wire and ground. These can be mounted
in a junction box close to the 7828.
Further protection may be offered by the use of an independently powered, fully isolated
RS485 to RS232 converter, which is essential whenever the Modbus output is
permanently connected to a PC. See section 3.4.3.
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