Hazardous Area Fire Protection by
Linear Heat Detectors - Installing LHDC
using Intrinsically Safe Barriers - Page 2
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Patol Analogue EOLs comprise a 22M resistor and meet the “Simple Apparatus” criteria; their parameters being effectively inconsequential in relation to LHD cable characteristics.
 
Patol Digital LHDC monitor circuits employ resistive EOLs with similar “conforming” characteristics.
Digital monitoring circuits using “capacitive” EOLs are not recommended, and other techniques such as EOL zener diodes require special consideration .
  

© Measurement Technologies Limited
 
 Barrier examples which can be supplied by Patol are MTL778 & MTL7778 units which are both dual channel 28V 600 ohm A.C. shunt diode safety barriers. The maximum permitted hazardous area cable parameters for these particular units with the channels combined in the hazardous area are:-
 
MTL7778
Combined Channels
EC Directive 94/9/EC (ATEX / BASEEFA)
FM
Group IIA
Group IIB
Group IIC
Groups A & B
Capacitance
2.15 uF
0.65 uF
0.083 uF
0.083 uF
Inductance
33.6 mH
12.6 mH
4.2 mH
4.00 mH
L/R ratio
333 uH/ohm
158 uH/ohm
42 uH/ohm
107 uH/ohm
 
Characteristics of Patol LHDC
Capacitance
Inductance
L/R Ratio
Analogue LHDC - Standard
< 350 pF/m
< 0.5 uH/m
< 2.5 uH/Ω
Analogue LHDC - Braided Armour
< 540 pF/m
< 0.7 uH/m
< 20 uH/Ω
Digital LHDC - Standard
< 120 pF/m
< 0.95 uH/m
< 5.5 uH/Ω
Digital LHDC Braided Armour
< 230 pF/m
< 0.95 uH/m
< 5.5 uH/Ω
 
It can be seen from the above that in the case of a 28V MTL 7778 Barrier employed for Analogue LHDC, without any interposing cable, the limiting parameter for the Group IIB gas area is LHD cable capacitance, which is directly related to length.
 
 In this example, the maximum permissible length would be:- 
 
0.65uF / 350pF    =   650000 / 350    =   1857 metres
 
From an Intrinsically Safe point of view 1857 metres may be acceptable, but with Analogue LHDC this zone length would most likely be further limited by the temperature length relationship for the installation. Ref. LHDC Data Sheet
 
 With higher voltage barriers the predominant factor is capacitance and as Digital LHDC has generally lower C/m, longer lengths may be installed than Analogue.
 
 Example: Even with the most onerous parameter which is for gas Group IIC 691m of Standard Digital LHDC may be installed. 0.083uF/120pF = 83000/120 = 691m
 
               
               
    Rectory Road, Padworth Common,
Berkshire UK - RG7 4JD
Tel : +44 (0) 118 9701701
Email : info@patol.co.uk
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