Hawker P7/I.S., Инструкции по эксплуатации и монтажу, Страница: 17 / 23

Page 17 of 22
6.2 Cable capacitance
Example of calculating cable capacitance
Normally it is the cable capacitance that will limit the length of the conductors.
Using the same scenario as above 4 x P1 and 2 x P2, for capacitance we ignore the ‘G’ so
are concerned with 6 cables.
The maximum capacitance permitted from drawing 2920 table for Group 11C for 6
systems is 0.143
µ
F. From Table 1, a 2.5mm
2
Steel wire armoured cable having a
capacitance of 0.43
µ
F per 1000m so –
The total cable length permitted is (0.143 / 0.43) x 1000 = 332m, or
332 / 6 = 55m per electrode circuit.
Similarly for two core 1.5mm
2
cable we have: -
(0.143 / 0.39) x 1000 = 366m, or 366 / 6 = 61m per electrode circuit.
6.3 Electrode capacitance
The electrodes will also contribute to the overall capacitance of the system by a tiny
amount, which is normally negligible; allowing up to 200pF between any two electrodes will
present no problem in normal practice. For example heavy-duty electrodes such as the
HPE8 when mounted 230mm apart may be up to 18m in length before this figure is
exceeded. Light duty electrodes such as the HPE7, mounted 75mm apart may be up to
15m in length, which is far beyond normal requirements.
6.4
Cable selection guide
There are many different types of commercially available cables all with different
specifications. If the cable length is restricted using S.W.A cable it is possible to use
instrument cable since these generally have a lower capacitance. A selection has been
given in this document for reference purposes, table 1, the user should check with the
manufacturers data that the cable is suitable, see also system section “Permissible
interconnecting cables”.
For functionality it is important not to exceed the units operational capacitance input as it
will affect its sensitivity, these can be found in the controllers technical specifications. The
overall system capacitance/inductance can either be calculated or directly measured using
a calibrated capacitance/inductance meter, which is preferable. This can be used to
confirm the total maximum table values in drawing 2920 have not been exceeded.