The Electrical Contractor Network

ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

Books, Tools and Test Equipment for Electrical and Construction Trades

Register Now!

Register Now!

We want your input!


2017 NEC and Related
2017 NEC
Now Available!

Recent Posts
Parking lot pole light swap....
by gfretwell
Yesterday at 11:04 PM
International Wire Colour Codes
by Tjia1981
Yesterday at 12:08 PM
Son of Sparky
by HotLine1
10/20/16 07:43 PM
Speaking of Plugmold ...
by gfretwell
10/17/16 02:37 PM
Broken battery charger? Check for cobwebs!
by gfretwell
10/17/16 02:30 PM
New in the Gallery:
12.5A through 0.75mm˛ flex (just out of curiosity)
Shout Box

Top Posters (30 Days)
gfretwell 13
HotLine1 7
ghost307 6
renosteinke 6
Potseal 4
Who's Online
1 registered (Tjia1981), 234 Guests and 4 Spiders online.
Key: Admin, Global Mod, Mod
Topic Options
Rate This Topic
#180730 - 09/07/08 06:04 PM phase failure protection
tradesmen Offline

Registered: 02/03/08
Posts: 10
Loc: shippensburg,pa
I have been called to a church that is having a problem with losing the "b" phase of a 120/208 volt system.This problem has happened 3 times in 2 weeks. Power company had a problem somewhere along the line which was causing the problem,but not at the church though.The church has 3 phase air conditioning equipment and are concerned about damage to the equipment do to low voltage.The service is 3 seperate 400amp fused disconnects,which are located in different parts of the facility.My question is,is there some type equipment that would shut down the power total if a phase is lost.better to have no a/c than run it on partial power
thank you for your in put

Work Gear for Electricians and the Trades

Work Gear for Electricians and the Trades
Work Gear for Electricians and the Trades
Arc Flash Clothing, Gloves, KneePads, Tool Belts, Pouches, Tool Carriers, etc. etc....

#180731 - 09/07/08 06:32 PM Re: phase failure protection [Re: tradesmen]
renosteinke Offline
Cat Servant

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
Yes, there is. And installing it might be easier than you think.

The newer starters have electronic 'overloads.' Apart from doing what the 'heaters' used to do, they also kick out for low voltage, or a phase imbalance. Motor is thus protected.

The best part? If you can stay with the same make, the new guts will fit in the old enclosures.

#180733 - 09/07/08 06:39 PM Re: phase failure protection [Re: renosteinke]
NORCAL Offline

Registered: 09/25/02
Posts: 805
What about a phase failure relay and tie it in to disable the A/C's?

#180734 - 09/07/08 08:40 PM Re: phase failure protection [Re: NORCAL]
sparkyinak Offline

Registered: 07/08/07
Posts: 1288
Loc: Alaska
Yep. A phase failure relay has a dry contact on it so it will open the contact. All you need to do is run the control circuit though the contact.
"Live Awesome!" - Kevin Carosa

#180735 - 09/07/08 09:02 PM Re: phase failure protection [Re: NORCAL]
JoeTestingEngr Offline

Registered: 11/04/05
Posts: 786
Loc: Chicago, Il.
I would go with the phase loss monitor (lotsa names for the same thing) as NORCAL suggested. I would also add a Time Delay Operate relay set to at least 5 minutes to prevent short cycling. The PLM looks for phase loss, imbalance, or reversal. A set of contacts from the PLM feed the timer at the HVAC control voltage. This wouldn't be necessary if the controls are already of a smart type that protect compressors. You could drive an indicator from the timer's NC contacts so that building maintenance staff can see that you are waiting for the power to be on long enough before turning on the AC.

#180746 - 09/08/08 09:11 AM Re: phase failure protection [Re: JoeTestingEngr]
JValdes Offline

Registered: 06/03/07
Posts: 308
Loc: South Carolina
A phase monitor is the way to go. But do not try to open the service, only the critical equipment like the air.
Use one set of contacts for the alarm.


ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals