MSA Senscient ELDS 1000 series, Руководство по эксплуатации

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Installation Design & Engineering
46
Senscient ELDS™
Rev 15
GB
r. The power consumption of each half of an ELDS 1000 / 2000 Series system is around 10W,
which is sufficient to create a significant voltage drop due to the current flowing through the
resistance of the field cabling. Where installation designers intend to ‘daisy chain’ the supply of
power to an ELDS 1000 / 2000 Series system by running the +24V and 0V connections to one
half of a system and from there to the other half - caution is advised. The voltage drop to the
first half of the system will be the product of the total system current and the cable resistance
to that point; whilst the voltage reaching the second half of the system will be still lower, further
increasing the current drawn by this half of the system. A ‘daisy chain’ arrangement should be
acceptable for systems installed within a few hundred metres of the control room, but for
greater distances the installation designer will need to calculate the voltage drops and
necessary cable resistances very carefully. Rather than use the same pair of conductors to
carry +24V and 0V to the system it may prove better to use a separate pair of conductors to
carry +24V and 0V to each half of the system.
s. All ELDS 1000 / 2000 Series units incorporate a power-up current limiting circuit. This circuit
ensures that even during power-up, the current drawn by a unit never exceeds the maximum
current that would be drawn by the unit when operating from the minimum supply voltage
(18V). See 3.3.5.
t. All ELDS 1000 / 2000 Series units incorporate a brown-out survival reservoir. Under normal
operating conditions this reservoir should be sufficient to enable the unit to continue operating
without being reset by a +24V power brown-out of up to 10mS duration. The installation
designer should therefore endeavour to ensure that any battery back-up or similar system
should detect a brown-out and restore the +24V power rail within 5mS.
u. Whilst signalling readings using a 4-20mA current loop is considerably more robust than
signalling readings using a voltage (which will normally drop between the field device and the
control room) it is not completely immune to cable losses. Where long cable runs are
employed between the field device and the control room it is possible for current to leak away
through the insulation, reducing the current reaching the control room and therefore the
apparent gas reading. (This can be misinterpreted as negative drift or warnings / faults being
signalled.) Current leakage is not usually a problem in high quality cables manufactured from
durable, resistant materials, but it can be a problem in lesser quality cables, especially after
such cables have seen several years of service. The installation designer is advised to ensure
that all of the insulation, protection and screening materials used in the construction of the
cables employed will resist the effects of chemical attack, corrosion, mechanical wear and
solar radiation that the cable is likely to experience over its anticipated service life. (Bear in
mind that if the outer insulation perishes, water will get through to any armour and screening
below, which will tend to corrode fairly rapidly. Once this has happened, you may have field
conductors running in a solution of metallic corrosion products surrounded by an earthed
conductor. This arrangement is ideal for bleeding current away from a 4-20mA loop, especially
in wet conditions.)
v. Where a low noise instrument (clean) earth is employed, this instrument earth should only be
connected to safety earth (usually dirty) at a single point on the site / installation. The location
and arrangement of this connection should be carefully engineered to minimise the
introduction of noise onto the instrument earth.
w. The terminal compartment of each ELDS 1000 / 2000 Series unit incorporates a single cable
entry on the underside of the unit. This arrangement minimizes the potential for water ingress
through the cable entry and discourages use of the terminal compartment for purposes for
which it is not well suited. If the installation designer wishes to ‘daisy chain’ units or marshal
the connections to several field devices, a separate junction box must be employed.
x. For compliance with ATEX requirements, the Ex d (flameproof) terminal compartment of ELDS
1000 / 2000 Series units can be entered by cables fitted with any suitable equipment certified
ATEX cable gland (not a component).
For further details of installation requirements see ATEX Control Drawings in section 9.2.