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Topic Title: Barriers
Topic Summary: Voltage drop across barrier
Created On: 18 June 2013 06:09 AM
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 18 June 2013 06:09 AM
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Posts: 31
Joined: 15 December 2006


I need help with this safety critical piece of equipment.

I have two Drexelbrook HHLA's installed on two different tanks on a petrochemical storage facility.

They are being fed from series 401-400 line monitoring receivers (barriers). These receivers are being powered from the excact same power supply.

The problem is that one of the HHLA's has intermittently stopped working. The electrician tested the voltage that is being outputted form the barrier to the HHLA probe and it is 10.22V, which is too low to power the HHLA correctly. When the voltage is this low there is no fault alarm or HHLA LED on the line monitoring receiver which makes it useless. If the electrician turns off the power supply to the line monitoring receiver and then powers up again the voltage increases to 13.32V and stays this way...... which then has everything working correctly, until it intermittently drops again at a later time.

When I replaced the barrier and HHLA with another one from another tank that did work before - it did exactly the same????

Please let me know what can cause this voltage drop......
 18 June 2013 10:05 AM
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Posts: 62
Joined: 28 September 2009

No surprises - if you've swapped the IS barrier and loop powered device (LA) then there is only the cable and PSU left....
Is the loop really voltage-driven? Most of these will be a current loop so measuring the voltage is a bit of a red herring.

EDIT to say that it's almost certainly a 4-20mA loop powered device...

Edited: 18 June 2013 at 01:52 PM by bartonp

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