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A simple guide to barrier selection

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Every intrinsically safe (I.S) circuit must be protected by a suitable intrinsically safe barrier.

There are two types of I.S. Barrier:

a) ZENER BARRIERS. These protection devices use a combination of zener diodes, resistors and fuses to protect the I.S circuit. Designed in the early 1980s, MTL700 series barriers must be replaced if the internal non-replaceable fuse blows. The more recently designed 7700 series have electronic fuses. Note that the negative rail of the compatible barriers listed below connects directly to earth; this can cause problems when used in conjunction with control panels fitted with earth fault monitoring facilities.

b) GALVANIC ISOLATING BARRIERS. These devices isolate the hazardous area from the safe area and using opto-isolator technology. They are fully resettable and do not contain fuses. The I.S. Circuit can be left floating or earthed at any one point thus avoiding potential earth loop faults which may be experienced using zener barriers. Although more expensive than zener barriers they provide a simpler engineering solution for I.S. circuit design.

Each of the barriers listed below will provide sufficient power to drive one sounder or beacon.

The barriers must be located in the safe area. They may be housed within the control panel or in a suitable enclosure. A table of enclosures and the quantity of barriers they can hold is given below.

ManufacturerModelZener BarrierIsolating barrier
E2Sall I.S modelsMTL 728+MTL5025
MTL7728+MTL5525
C & SYO4 ISA & YO5 ISAMTL 728+MTL5025
MTL7728+MTL5525
MEDCDB5MTL 728+MTL5025
MTL7728+MTL5525
DB8MTL5021
APOLLOSERIES 60 & ORBISMTL 5061*
SERIES 95REFER TO OUR SALES OFFICE
DISCOVERYREFER TO OUR SALES OFFICE
* 2 Zones each containing 5 detectors can be protected by 1 barrier

Barrier enclosures

Barrier enclosures come complete with mounting rails, fixings, I.S. circuit warning labels. The quantity of barriers each enclosure will hold varies by barrier type as per following table:

ENCLOSURE5000 SERIES5500 SERIES7700 SERIES700 SERIES
DX070445n/a
DX170101013n/a
DX430262633n/a
MT2n/an/an/a2
MT5n/an/an/a5
MT12n/an/an/a12
MT20Nn/an/an/a20