Honeywell Searchpoint Optima Plus Operating Instructions Manual page 27

MAN0905_Issue 7_07/17
In addition, consider the following points: -
General recommendations
1. In general, correctly engineered star earthing arrangements minimise earth current crosstalk and noise,
improving the reliability and performance of instrumentation.
2. The use of a single, screened / shielded cable for each field device ensures good screening / shielding
and reduces crosstalk.
3. Low noise instrument earth should only be connected to protective earth at a single point on a site /
installation. This connection should be made in such a manner that it does not introduce noise onto the
instrument earth.
Field Cabling
1. The entire length of the field cabling connected to a unit should be screened / shielded. The screen /
shield should be connected to a low noise instrument earth at one end.
2. The screens / shields of field cabling must not be connected in a manner that creates earth loops or that
will result in the screens / shields carrying large currents from heavy plant or equipment.
3. If armoured cable is being used it is necessary to prevent the armour becoming connected to the screen
/ shield.
4. For installations where the field cable conductors run through conduit or armour connected to protective
earth, the conduit or armour should not be considered as a sufficient screen / shield. A separate cable
screen / shield, connected to a low noise instrument earth should be employed.
Maximum voltages
1. Any earthing arrangement employed should ensure that the maximum peak voltage between the unit's
case earth and any field cable conductor is less than 500 V. This includes conditions where transient
surge voltages are generated by lightning or the switching of heavy electrical plant. Earthing
arrangements that employ multiple protective earths or earth grids significantly increase the likelihood of
large transient surge voltages being generated between a unit's case earth and the field cable
conductors. If it is suspected that voltages between the unit's case earth and any field cable conductor
may exceed 500 V it is strongly recommended that surge suppression devices be installed close to
the unit.
Note: Suitable, certified surge suppression devices include MTL's TP-48-I-NDI and Pepperl & Fuchs'
FN-LB-1. Surge suppression devices should be checked regularly and replaced if necessary.
Interference and noise
1. For installations in Europe, all electrical equipment connected to the system should comply with
EN50270. For non-European installations, electrical equipment connected to the system should comply
with applicable national or international EMC standards.
2. Any electrical interference induced onto the conductors of a 4 – 20 mA loop by the installation must be
kept below the levels necessary to comply with the requirements of EN60079-29-1 or other applicable
gas detector performance requirements / standards. In practice, this means that peak noise currents
induced on the current loop should be no greater than ±0.25 mA.
3. The 0 V rail of the control card/control system is often directly connected to one side of the 4-20 mA
current sensing resistor. Electrical noise on this rail is therefore directly connected to the 4-20 mA
input. In order to avoid additional noise being induced on the 0 V rail, it should not be commoned with
the protective earth, which frequently carries a high level of electrical noise.
4. Ideally, the 24 V supply reaching units should be free from large transients, fluctuations or high frequency
noise.
4. Electrical Installation
Searchpoint Optima Plus with optional HART
27
output
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Operating Instructions
2108M0550