Single Phase preventer working and installation Procedure

Phase preventer are used in industries are having following advantages:-

1.       They uses to protect the motors and electrical systems in case of one/ two phase failure in three phase power system i.e. they can protect the system from single phasing.
2.       They protect the system in case phase sequence gets changed and phase sequence protects the system so that motors could not start operating in reverse direction.
3.       They also protect the system when voltage level falls below a particular level.
Phase preventer is also named as single phase preventer.
Installation of Phase preventer in electrical systems:-
Phase preventer consists of following:-
1.       Terminals for Three phase supply
2.       Terminals for auxiliary supply
3.       NO/ NC contact for interlocking
Connections for the relay as below:-

Single phase preventer connections

There are few additional settings provided on Phase preventer are:-
(i)                  Under voltage setting :- Do the Under Voltage setting to minimum
(ii)                Under voltage delay settings: Keep the Under Voltage Delay to minimum
(iii)               Unbalanced Voltage Settings:- Keep percentage Unbalanced to maximum

Now connect the three phase power supply at three terminal for three phase power supply
Now provide single auxiliary power supply at auxiliary power supply points.
Now finally connect Interlocking connections at Phase preventer so that when there is phase failure motor or required machine get tripped

Now turn ON power supply and Phase preventer get energized and if RED light on phase preventer get green then phase sequence is correct and voltage in all phases is above minimum under voltage setting.

If after energizing the Phase preventer LED light on Phase preventer glows RED then this means phase sequence needed to be changed of three phases Input supply or main supply. After changing phase sequence light made to glow green.


Also when Green light is on display of phase preventer then NO and NC will remains the same .  When phase sequence get changed then NO becomes NC and NC becomes NO so interlocking motor or machine get tripped.

Under Voltage Trip point setting:-
To protect equipment from under voltage it is always recommended to keep under voltage setting to as minimum as possible. If you don’t know the settings then adjust the under-voltage setting knob until LED starts glowing red. Now Slowly adjust the knob to setting in such a way that relay starts glowing green. Now at that setting it will ensured that voltages are under acceptable limits.

Under Voltage Delay Settings:-
The delay set in this setting is the maximum time for which under voltage can exist in system before causing tripping of motor or machine.
Settings can be done in such a way that if setting is done too low then it may cause nascence tripping in the system. If setting is done too high then it may cause damage to motor or machine and purpose of installation get defeated.

Voltage Unbalance setting:-
Voltage unbalance is another setting provided on the relay. In any system maximum allowable unbalanced voltage is not specified. But it generally acceptable to level of (+-) 5% depending upon allowable limits specified on motor or machine terminal plate.

In every system there will not be perfectly balanced voltage in system. So Unbalanced voltage setting to be done accordingly, too low settings will cause unwanted interruptions and too high settings will may cause damage to motor or machine.

Another way of doing the setting is to first adjust the knob for unbalanced voltage setting until relay starts glowing red , after that now slowly adjust the knob in such a way that LED starts glowing green.

As per NEMA %age Unbalanced Voltage= (Maximum Deviation from Average voltage/ Average Voltage)X 100

Where average voltage= (L1+L2+L3)/100

After doing all settings relay LED was glowing green.



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