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Relay failure modes

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gilles.thesee(at)free.fr
Guest

Posted: Sun Jan 08, 2017 2:05 am Post subject: Relay failure modes

Hi all,



Is there such a thing as a typical, or most likely relay failure mode ?



For instance "failure to operate when the coil is energized", 

"uncommanded contact opening", etc. ? We are speaking of "ordinary" relays.



Let's say we have a load permanently connected to the normally closed 

contacts, and the relay is occasionally energized to open the circuit. 

Is failure likeliness on the same order as that of the contacts of a 

switch, for instance ? Or does such a relay significantly impair the 

circuit reliability vs an ordinary switch ?



Thanks in advance for any facts or opinions,



-- 

Best regards,

Gilles

http://contrails.free.fr

http://lapierre.skunkworks.free.fr

cbirdsall6(at)cox.net
Guest

Posted: Sun Jan 08, 2017 5:26 am Post subject: Relay failure modes

My experience: 



Most common relay failure mode by far is failure to "set" - whether by mechanical or electrical failure inside the relay, the contacts fail to open or close as designed when current is applied to the coil. Often begins manifesting itself as intermittent operation (sometimes works, sometimes not).



Second to that is not an actual relay failure, but a failure in the installation. The result is the same (failure of the relay to actuate), but the cause is usually a wiring or mechanical fault introduced by the person that installed it. Bad soldering, incorrectly terminated wires, insufficient ground, not securing plug-in type relays, etc.  Also often begins with intermittent operation.  



Third and low in percentage of failures would be failure of the relay to "reset" after the coil current was removed (stuck "on").



Can't say as I've ever seen a relay actuate without input to the coil. Whenever I've run across a relay that's actuating when it's not supposed to be, it's always been a fault in the controlling circuit. I suppose it's possible under high g or heavy vibration to mechanically move the contacts. 



Chuck



On Jan 8, 2017, at 4:03 AM, GTH <gilles.thesee(at)free.fr> wrote:






Hi all,



Is there such a thing as a typical, or most likely relay failure mode ?



For instance "failure to operate when the coil is energized", "uncommanded contact opening", etc. ? We are speaking of "ordinary" relays.



Let's say we have a load permanently connected to the normally closed contacts, and the relay is occasionally energized to open the circuit. Is failure likeliness on the same order as that of the contacts of a switch, for instance ? Or does such a relay significantly impair the circuit reliability vs an ordinary switch ?



Thanks in advance for any facts or opinions,



-- 

Best regards,

Gilles

http://contrails.free.fr

http://lapierre.skunkworks.free.fr

user9253

Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan

Posted: Sun Jan 08, 2017 7:16 am Post subject: Re: Relay failure modes

It takes a lot more magnetic force to pull a relay armature in than it does to hold it in the pulled position.  In fact, just residual magnetism in the coil core could be enough to hold the armature pulled in, even after the coil has been de-energized.  To prevent that, a thin layer of non-magnetic material is bonded to the armature.  If that thin layer wears away or falls out, the relay could remain in the pulled-in position after the coil has been de-energized.

  At work many years ago, a machine operator complained that a 5 HP, 3 phase, 480 volt motor started up all by itself after he had previously shut it off.  Another electrician verified the operators observation, but could not find the problem.  He asked me to help.  It turns out that the motor did not actually stop running.  But it appeared that it did because the run-indicator light went out when the operator pressed the stop button.  I measured the coil voltage of the 3 phase contactor and it was zero.  So the contactor never pulled in all by itself for no reason, it just never dropped out.  Replacing the contactor fixed the problem.  Examining the contactor back in the shop, it repeatedly stuck closed when manually pulled in.  I do not know if was residual magnetism or atmospheric pressure (like a suction cup) that held the large area mating surfaces together.  The mating surfaces were slightly oily.  This was a highly unusual failure.  I only saw it happen that one time.

  So yes, it is possible for relays to get stuck in the closed position.  But it is not very common.  If I wanted to use relays in my airplane, I would not let fear of relay failure deter me.  A relay is less likely to fail than the fuel system is.  If the relay controls a critical circuit, then have a backup plan.

_________________
Joe Gores

MMiller

Joined: 07 Jul 2016
Posts: 8

Posted: Sun Jan 08, 2017 7:26 am Post subject: Re: Relay failure modes

Gilles,



Often the failure of a relay can be traced to a design choice. On a single pole relay the most common failure modes (in no special order);

Open coil (won't pick)

Shorted coil (blows fuses)

Welded contact (fails to break)

Contact fails to make (dirt)

Mechanical binding



Relays are generally used to drive loads greater than the rating of the switch piloting the relay or when multiple signals (poles) are needed. Comparing equally rated switches and relays, one disadvantage of a relay is that it adds one more step to the process, a power failure in the coil circuit and it won't pick.  Other than that, It doesn't matter if you use a switch or a relay, it's more important that the device you choose meets the requirements of the circuit.  A properly selected relay or switch will likely last then life of the design, trouble free.



When choosing a contact, bigger isn't better. As an extreme example, don't use a contactor in a telecom circuit.  Also, not all loads are equal, when driving an inductive load, manage the kickback and de-rate, when driving lamps, the cold filament can be 10 times rated lamp current. 



Mike

Last edited by MMiller on Sun Jan 08, 2017 3:04 pm; edited 1 time in total

user9253

Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan

Posted: Sun Jan 08, 2017 8:01 am Post subject: Re: Relay failure modes

Vern Little has lots of relays in his plane.  Below is a link to his electrical schematic.



http://www.vx-aviation.com/sprocket/photos/panel_elec/schematics/Rocket_electrical-C1.sch

_________________
Joe Gores

Peter(at)sportingaero.com
Guest

Posted: Sun Jan 08, 2017 3:23 pm Post subject: Relay failure modes

What is an sch file? What app can open these?



Peter



--

wgreenley

Joined: 09 Jan 2010
Posts: 100
Location: Dowagiac, MI

Posted: Sun Jan 08, 2017 3:34 pm Post subject: Relay failure modes

Can open with ExpressSCH, free at expresspcb.com



On Sun, Jan 8, 2017 at 6:20 PM, Peter Pengilly <Peter(at)sportingaero.com (Peter(at)sportingaero.com)> wrote:

[quote]--> AeroElectric-List message posted by: "Peter Pengilly" <Peter(at)sportingaero.com (Peter(at)sportingaero.com)>

 

 What is an sch file? What app can open these?

 

 Peter

 

 --

larkrv10(at)gmail.com
Guest

Posted: Sun Jan 08, 2017 3:40 pm Post subject: Relay failure modes

Peter the software comes from Express PCB, https://www.expresspcb.com/





I've used it to do all my elementary drawings.





Rick

#40956

Southampton, ont

[url=https://www.google.ca/?gws_rd=ssl#][/url]

On Sun, Jan 8, 2017 at 6:20 PM, Peter Pengilly <Peter(at)sportingaero.com (Peter(at)sportingaero.com)> wrote:

[quote]--> AeroElectric-List message posted by: "Peter Pengilly" <Peter(at)sportingaero.com (Peter(at)sportingaero.com)>

 

 What is an sch file? What app can open these?

 

 Peter

 

 --

user9253

Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan

Posted: Sun Jan 08, 2017 3:42 pm Post subject: Re: Relay failure modes

The Express.sch program is similar to Excel in that there can be more than one tab.  After opening Vern Little's schematic, click on the various tabs at the bottom of the page.

_________________
Joe Gores

n233ee(at)gmail.com
Guest

Posted: Sun Jan 08, 2017 4:43 pm Post subject: Relay failure modes

Has anyone used this on an iPad?Â 



On Jan 8, 2017 5:49 PM, "user9253" <fransew(at)gmail.com (fransew(at)gmail.com)> wrote: 	  Quote: 	
		  --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com (fransew(at)gmail.com)>

 

 The Express.sch program is similar to Excel in that there can be more than one tab.Â  After opening Vern Little's schematic, click on the various tabs at the bottom of the page.

 

 --------

 Joe Gores

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=464882#464882

 

 

 

 

 

 

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nuckolls.bob(at)aeroelect
Guest

Posted: Sun Jan 08, 2017 7:57 pm Post subject: Relay failure modes

At 04:03 AM 1/8/2017, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: GTH <gilles.thesee(at)free.fr>



 Hi all,



 Is there such a thing as a typical, or most likely relay failure mode ?



 For instance "failure to operate when the coil is energized", "uncommanded contact opening", etc. ? We are speaking of "ordinary" relays.



 Let's say we have a load permanently connected to the normally closed contacts, and the relay is occasionally energized to open the circuit. Is failure likeliness on the same order as that of the contacts of a switch, for instance ? Or does such a relay significantly impair the circuit reliability vs an ordinary switch ?



 Thanks in advance for any facts or opinions, 	


   Relay failures are rare . . . as are switch

   failures. Relays are simply remotely operated

   switches having manual actuators replaced with

   electro-magnetic actuators.



   If one does measurement/analysis of contact performance

   and the effects of environmental stress for

   manual switches vs. electro-mechanical relays,

   they are virtually identical. Contacts are

   made from the same materials. They 'bounce'

   on closure. Contact erosion and tendency to

   'stick' as a byproduct of metal-transfer in

   the electric arc are the same.



   The only failures I had occasion to study

   on TC aircraft had foundation in poor

   application of the relay to intended task.

   I worked one relay failure program on a bizjet

   that took several years of sand-sifting to

   identify root cause. The relays were being

   used in a very benign environment (pressurized

   cabin) and well within their electrical . . .

   but were known to 'stick'. Single sticks

   would disable a trim system. Double sticks

   would produce a trim runaway.



   Yet, all the 'failed' relays showed minimal

   wear and tear . . . and would test normally

   in a production test fixture.



   Relays are, as a class of product, no more

   prone to failure than their remote cousins

   on the panel.  I have operated three vehicles

   to a cumulative total of more than 400K miles.

   All three machines have a dozen or more relays

   scattered about various systems. I've experienced

   a range of mechanical failures due to end of life

   wear out . . . but haven't replaced a single

   relay.



   Relays, like switches, have design lifetimes

   in the tens of thousands of operations at

   rated loads. Few relays in light aircraft see

   more than a few hundred cycles per year . . .

   and none are stressed to their maximum rated

   loads.



   We've studied some contactor failures due

   to manufacturing error . . . a few more due

   to electrical abuse (starter contactors down

   stream of a soggy battery). But for the most

   part, kept a cap-checked and charge battery

   installed and even that risk drops dramatically.

   Others were subject to moisture ingress due

   to location and poor choice of installation

   orientation.



   You are probably more likely to lose a relay's

   functionality due to a bad crimp on a mating

   terminal than from failure within the relay.

   The vast majority of failures I've investigated

   on heavier iron had simply reached the end of

   their service life . . . they were worn out.



 

 

   Bob . . .

gilles.thesee(at)free.fr
Guest

Posted: Mon Jan 09, 2017 8:44 am Post subject: Relay failure modes

Le 09/01/2017 à           04:53, Robert L. Nuckolls, III a écrit :

         

      	  Quote: 	
		   Relay failures are rare           . . . as are switch

             failures. Relays are simply remotely operated

             switches having manual actuators replaced with

             electro-magnetic actuators.     	
     

     Bob and all who reponded,

     

     Thanks a lot for the facts and detailed explanations.

     I do know that simpler is better, that a part not installed cannot     fail, etc.

     But concerning the so peculiar Rotax 912iS engine, my partners     decided - and I do agree - that preventing pilot's errors is of     highest priority.

     So we are studying the possibility of grouping some functions or     sequences.

     

     The idea behind my relay question was powering an appliance via a     normally closed relay, and energize that relay (open the contacts)     only for testing once or twice during ground runup.

     

     Still more funny questions to come !

     Thanks again,

     -- 

       Best regards,

       Gilles

       http://contrails.free.fr

       http://lapierre.skunkworks.free.fr

user9253

Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan

Posted: Mon Jan 09, 2017 12:33 pm Post subject: Re: Relay failure modes

A momentary push button switch could energize a relay for testing with no chance of the pilot leaving the switch in the wrong position.

_________________
Joe Gores

gilles.thesee(at)free.fr
Guest

Posted: Mon Jan 09, 2017 2:49 pm Post subject: Relay failure modes

Le 09/01/2017 à 21:33, user9253 a écrit :

 	  Quote: 	
		  

 A momentary push button switch could energize a relay for testing with no chance of the pilot leaving the switch in the wrong position.



Joe,
	




Yes, that's the idea.

-- 

Best regards,

Gilles

http://contrails.free.fr

http://lapierre.skunkworks.free.fr

reaper

Joined: 12 Jun 2016
Posts: 8

Posted: Sun Feb 26, 2017 3:36 pm Post subject: Re: Relay failure modes

 	  nuckolls.bob(at)aeroelect wrote: 	
		  

 

   We've studied some contactor failures due

   to manufacturing error . . . a few more due

   to electrical abuse (starter contactors down

   stream of a soggy battery). But for the most

   part, kept a cap-checked and charge battery

   installed and even that risk drops dramatically.

   Others were subject to moisture ingress due

   to location and poor choice of installation

   orientation.



 

   Bob . . . 	


I'm installing my battery contactor (Cole Hersee) 'cap down'. 



It seems extremely unlikely that G loading would 'open' the contactor, 

but my question is what would be the system repercussions 

should a battery contactor open while the alternator was 

under normal system load? 



Would the crowbar overvoltage protect the avionics?

nuckolls.bob(at)aeroelect
Guest

Posted: Mon Feb 27, 2017 8:54 am Post subject: Relay failure modes

 	  Quote: 	
		  

 I'm installing my battery contactor (Cole Hersee) 'cap down'. 	


   That's fine.



 

  	  Quote: 	
		  It seems extremely unlikely that G loading would 'open' the contactor, 	


   Change 'unlikely' to 'impossible'

  	  Quote: 	
		   

 but my question is what would be the system repercussions 

 should a battery contactor open while the alternator was 

 under normal system load? 	


   Not a thing. You probably won't know the

   contactor is open until you turn on the

   master during next pre-flight (depending

   on sequencing of switches).



  	  Quote: 	
		  Would the crowbar overvoltage protect the avionics? 	


   If the alternator were seriously unstable

   sans battery . . . but this is unlikely.

   If you have Klieg Lights on the wings or an

   electro-hydraulic landing gear system, then

   you may stall the system when those accessories

   are activated . . . but unless you make some

   LARGE changes in electrical system demands

   before landing, you'll probably be unaware

   of the loss of a battery contactor.



 

   Bob . . .

reaper

Joined: 12 Jun 2016
Posts: 8

Posted: Mon Feb 27, 2017 9:36 am Post subject: Re: Relay failure modes

Great



I'm using a 2-10 per Z-11P for Batt/Alt so I guess 

I would notice on shutdown.



Thanks!



dave

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