Will 2 out of 3 Phases Trip a 3 Phase Breaker?

[Peter-L]

Hello,

We've had an issue with a 3 phase piece of equipment that was tripping the 3 phase breaker as soon as it was turned on. On investigation (by a certified electrician) he said it was the equipment at fault. I raised a return of the equipment and the new one arrived today. To be able to return the 'faulty' equipment I had to remove the commando socket from it and its associated cabling from the faulty one, on removing the access panel I found that the cable going into the phase 1 junction block was loose.

Would this loose cable have cause the breaker to trip, and if so would this have saved the hardware from being damaged when only receiving 2 out of 3 phases?

Any guidance much appreciated folks!

 

[Peter-L]

For clarification the Phase 1 cable was in the junction block but the screw hadn't been tightened.

 

[Deservices]

It's possible. As 2 phasing causes high current on the remaining phases. Might be a coincidence with the work being done, cables moved etc, but would be nice to think he checked that.

 

[Deservices]

Then yes, loose wire will cause 2 phasing and with trip the breaker. Would of thought it would be burned alittle thou if its been used for a while.

 

[Phillcbr400]

It could just be from when the spark was testing.
Any signs of arcing on that terminal?

 

[Engineer54]

Any fault on one of the 3 phases, can trip a 3 phase breaker. As for your situation, no-one can give you a definite answer as to whether this was your problem or not. Just make sure that on delivery of the replacement unit, all connections are adequately tightened before putting into service....

 

[Peter-L]

It's possible. As 2 phasing causes high current on the remaining phases. Might be a coincidence with the work being done, cables moved etc, but would be nice to think he checked that.

The strange this was it was at 8pm at night, there would have been no work going on and the Unit had been in service for 2 weeks. I'll also check for arcing

It could just be from when the spark was testing.
Any signs of arcing on that terminal?

I'll check for arcing but the testing was only done pre-installation and post failure/tripping.

Any fault on one of the 3 phases, can trip a 3 phase breaker. As for your situation, no-one can give you a definite answer as to whether this was your problem or not. Just make sure that on delivery of the replacement unit, all connections are adequately tightened before putting into service....

Its not my job and im not qualified to do so but I will definitely be checking if the connections are tight prior to service. As there is now a high chance we will be billed for the supposedly failed unit (approx £6k)

 

[Engineer54]

Its not my job and im not qualified to do so but I will definitely be checking if the connections are tight prior to service. As there is now a high chance we will be billed for the supposedly failed unit (approx £6k) ​

Not necessarily, if the connection that was found loose, was within the machine itself!! Does the manufacturer not have a service provider that could have been directed by the manufacturer to check out the machine, prior to any replacement being called for?? ​

 

[Darkwood]

Then yes, loose wire will cause 2 phasing and with trip the breaker. Would of thought it would be burned alittle thou if its been used for a while.

Be careful how you word this, you may know what you mean here but OP isn't an electrician. Loss of a phase through a three phase mcb alone will not cause it to trip whether from the supply side or broken wire load side, having said that and as you noted in previous post it may cause excessive current rise in other phases and this will depend on the load and its characteristics i.e. if a 3ph motor connected then it will 2phase on start-up giving high current spike on the stalled motor until it is detected by the motor overload or mcb or if poorly designed install/control circuit then until the motor catches fire and shorts

 

[ayjay]

could you get the existing machine reinstalled correctly then try it again?

 

[Peter-L]

Not necessarily, if the connection that was found loose, was within the machine itself!! Does the manufacturer not have a service provider that could have been directed by the manufacturer to check out the machine, prior to any replacement being called for??

The connection that was loose was where the electrician connects the single or 3 phase connection to the distribution panel. The manufacturer asked me to take photos but after reviewing them it doesnt show the cable was loose and it does look like it was in the terminal correctly from the angle of the photo. I just took the electricians word at face value when he 'checked' everything over post failure.




I've also checked the terminal block and cant see any scorching or melting of the plastic nearby. Subsequently I've found that the breaker was rated at 50A and should have been a 63A. That being side the monitoring we have on our other units (Sorry I never mentioned these are Server UPS with batter packs), that would be drawing approx the same load (allbeit across 3phases) where about - 28A and 44A (these have also taken the load of the failed unit so are higher than normal)

could you get the existing machine reinstalled correctly then try it again?

Potentially yes though every shift of these units are 140kg being lugged around and would require out of hours work to be performed, I just figured that we have a new unit onsite we might aswell use it just on the off chance the original did have a fault?

 

[Darkwood]

Just to clarify are you trying to say the qualified Electrician who came out to investigate this issue didn't check the plug connections and associated connections to the machine before he decided to leave... i would have thought this the most basic of checks especially if you the customer actually find the problem yourself?

I would be contending the callout charge, he should have checked to confirm 3ph at the machine as well as the integrity of the connection... anything past the incoming connection block would be the responsibility of the suppliers... check the installation instructions and i can almost guarantee that if the customer has to provide the connection to the terminal block in the machine then it will state to check all connections are tight.

Who fitted the flex and plug originally?
If it was fine wire cable were the correct lugs/ferrels fitted etc etc ....many questions to ask before we point a finger of blame here.

 

[Marvo]

Subsequently I've found that the breaker was rated at 50A and should have been a 63A.

The MCB (circuit breaker) is rated according to the cable it is supplying...not according to the equipment connected. The MCB isn't necessarily incorrectly sized, it may be the cable is undersized for a 63 Amp load.

(Sorry I never mentioned these are Server UPS with batter packs), that would be drawing approx the same load (allbeit across 3phases) where about - 28A and 44A (these have also taken the load of the failed unit so are higher than normal)

If the machine is a UPS then it would detect supply voltage instability caused by a loose terminal connection. If the terminal was loose for any time during service it would have constantly been running in battery back-up power mode. A UPS should be pretty much immune to any power supply issues, even a missing phase wouldn't cause damage.

 

[Peter-L]

Just to clarify are you trying to say the qualified Electrician who came out to investigate this issue didn't check the plug connections and associated connections to the machine before he decided to leave... i would have thought this the most basic of checks especially if you the customer actually find the problem yourself?


I would be contending the callout charge, he should have checked to confirm 3ph at the machine as well as the integrity of the connection... anything past the incoming connection block would be the responsibility of the suppliers... check the installation instructions and i can almost guarantee that if the customer has to provide the connection to the terminal block in the machine then it will state to check all connections are tight.


Who fitted the flex and plug originally?
If it was fine wire cable were the correct lugs/ferrels fitted etc etc ....many questions to ask before we point a finger of blame here.

The electrican that came out to investigate post the failure was the same one that wired it up in the first place, I can only think that he assumed it was correct. But yes I (the customer) found the problem when the cable came loose in my hand (thankfully commando socket was disconnected).


The cable doing into the junction box is H07RN-F and the strands were just twisted to go into the junction box.


Heres a photo of the connections and of the flex etc. The left and right connections were both loose enough that they came out in my hand.

The MCB (circuit breaker) is rated according to the cable it is supplying...not according to the equipment connected. The MCB isn't necessarily incorrectly sized, it may be the cable is undersized for a 63 Amp load.






If the machine is a UPS then it would detect supply voltage instability caused by a loose terminal connection. If the terminal was loose for any time during service it would have constantly been running in battery back-up power mode. A UPS should be pretty much immune to any power supply issues, even a missing phase wouldn't cause damage.

Yea I agree, though the runtime for these units with our current load is approx 30mins and this happened outside of office hours when nobody could even have been near the unit to cause the cable to work loose/arc.

 

[Darkwood]

Finewire cable including the one fitted to your machine is required by regulation to be terminated into a connector either designed specifically for fine wire or it would in your case require feral crimps, this is to limit the chance of poorly made terminations caused by the wire in the case of connectors riding up the side of the thread and not been clamped fully and limit the chance of overtightening cutting through the strands common with some brands of connectors due to the grub screw shape.

Now most electricians out their really never bother with this rule or are unanware of it but i build control systems and do mainly industrial so even flexing a cable into a 3ph plug would be ferel crimped and especially on standard strip connectors, loose or poor connections on my job dont just stop the kettle working they cause downtime and financial loss.


Regulation 526.8.1 covers this and following reg's dont allow the other option of tinning of the cables due to the screw terminals in use.

Just note that we are speculating this was the cause of your problem but it may not be the cse ut either way he should have followed this regulation.