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Access Control and Earth Ground Fault Detection Featured

Friday, 27 February 2015 14:10 Written by 

Earth Ground Fault Detection was developed for the Fire Alarm industry to give enhanced fault detection on NAC loops.  Although Earth Ground Fault Detection is not currently required in access control applications, it can still be a useful feature - provided your system is compatible.

Earth Ground Fault

What is Earth Ground Fault Detection?

Despite what its name might imply, Earth Ground Fault Detection does not detect a missing earth ground connection to your power supply.  Instead, it detects a connection between earth ground and any positive or negative output circuit in the system.  This includes any part of the system that is common grounded with the power supply.  If the power supply with earth ground detection is powering an access control panel, any earth ground faults on the access panel circuits (readers, etc) will show on the power supply.  If your access control panel is then common grounded with your 24VDC CCTV system's DVR, any faults on the CCTV system would also show on your power supply.  Because of this, it is important to be aware of what is connected to your system - especially when troubleshooting.

How Does Earth Ground Detection Help?

An earth ground fault does not mean you necessarily have an immediate problem.  It is simply a warning that one leg of a circuit somewhere is either partially or fully shorted to earth ground.  Usually, a single short to earth ground causes no problem in the operation of the system.  However, a second short to earth ground - even on a completely unrelated circuit, can cause a direct short circuit, leading to blown fuses, erratic operation, or even a fire.

When should I not enable Earth Ground Fault Detection?

Earth Ground Fault Detection should only be enabled on one piece of equipment within the system.  In access control, this is usually the power supply.  If the access system is common grounded with the fire system, the FACP may already have earth ground detection enabled.  Also, if there are multiple power supplies connected to the access system, only one of these supplies should have Earth Ground Fault Detection enabled - this includes two supplies within the same enclosure.

In many access control applications, even if you follow all of the rules and none of your wiring or load devices is shorted to earth ground, you may still show an Earth Ground Fault.  This is usually caused by access control panels that don't "play nice" with earth ground fault detection.  As stated above, Earth Ground Fault Detection is not a requirement in access control.  Many access panels require shielded communications circuits and the panel manufacturers connect these shields to earth ground and DC ground for better shielding or other RF noise or transient requirements.  In these cases, the only option is to disable the Earth Ground Fault Detection entirely.

Troubleshooting Earth Ground Faults

For more information, our application note AN-32 (Troubleshooting Earth Ground Faults) goes in-depth on how earth ground fault detection works and the best method of troubleshooting earth ground fault conditions.  And as always, our Technical Support Department is always here to help.

Last modified on Monday, 02 March 2015 09:35
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Mike

LifeSafety Power Technical Support and Applications
mbone@lifesafetypower.com
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Website: www.lifesafetypower.com