Why didn't the overload trip?

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I have a 3-phase motor connected to a motor starter (contactor/overload). The FLA is 3.5A and everything is connected with 14ga THHN wire.
I found that L3 was not connected at the output of the overload. The result was the L1 fuse was obliterated, the L2 fuse was blown, and the L3 fuse tested as good. The fuses were 6A slow-blow, class CC fuses. The overload is adjustable and is set for 5A.
Why didn't the overload trip before the L1 fuse disappeared in an obvious single-phase condition?
 

GoldDigger

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I have a 3-phase motor connected to a motor starter (contactor/overload). The FLA is 3.5A and everything is connected with 14ga THHN wire.
I found that L3 was not connected at the output of the overload. The result was the L1 fuse was obliterated, the L2 fuse was blown, and the L3 fuse tested as good. The fuses were 6A slow-blow, class CC fuses. The overload is adjustable and is set for 5A.
Why didn't the overload trip before the L1 fuse disappeared in an obvious single-phase condition?
The overload is a thermal element or other time*current profile, and usually does not contain a phase loss detector.
The thermal mass of the motor would likely delay its trip beyond the fuse time constant. Especially on a single phase condition where the thermal load on the motor from two windings will be comparable to only 2/3 of the normal overload condition, while each fuse sees only its own line current.
 

Tony S

Senior Member
What make of overload unit, many have single phase detection built in.

For a fuse to be ?obliterated? suggests something more than a simple overload problem. In a really bad fault I?ve had the O/L unit destroyed, there?s more to this that you?re telling us.
 
Tony S - I don't know what to add. This happened yesterday while I was out.

My concern is that the overloads are set at 5A. The fuses were 6A. Shouldn't the OL trip before the fuses blow?
 

iwire

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My concern is that the overloads are set at 5A. The fuses were 6A. Shouldn't the OL trip before the fuses blow?

No.

Overloads react slowly, they are to protect motors from overheating.

On the other hand the fuses typically react faster.

So if the current was to spike quickly we expect the fuse to open before the overloads.
 

Tony S

Senior Member
Tony S - I don't know what to add. This happened yesterday while I was out.

My concern is that the overloads are set at 5A. The fuses were 6A. Shouldn't the OL trip before the fuses blow?

Thermal overloads should be set to 100% 3.5A in this case, the fuse is to protect the cable not the motor and should be rated to match your codes and current ratings.

Have you tested the motor and got continuity and insulation resistance values? If so what are they?

There a good chance the motor has survived and the fuses failed simply through fatigue.
 

Jraef

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Thermal overloads should be set to 100% 3.5A in this case, the fuse is to protect the cable not the motor and should be rated to match your codes and current ratings.

Have you tested the motor and got continuity and insulation resistance values? If so what are they?

There a good chance the motor has survived and the fuses failed simply through fatigue.
I agree, the OL was set wrong, should have been set at 3.5A. Many many people do this wrong, because they read in the NEC that the OL should not allow more than 125% current to flow to the motor (with a 1.15SF). But what they fail to do is read the INSTRUCTIONS for the OL relay, which tell you that the relay is ALREADY calibrated for 125% as the pick-up point of the trip curve, so if you SET it for 125% AGAIN, you are at 125% of 125%... too high to protect the motor.

As for single phasing, it all depends on the LOAD on the motor. If a motor is FULLY LOADED, and a phase is lost, the current on the other two will increase by about 50%, which because it is 25% higher than the pick-up point, trips the OL eventually. But if the motor is only 75% loaded at the time the phase is lost, then the current only increases to 112.5% and the OL will NOT trip, because it is below the 125% pick-up point.

In your case you had the double whammy: the pick-up point was ALREADY at 125% of 5A, so 6.25A. which compared to FLC is 179% of the motor nameplate. So even IF the motor was fully loaded when the phase was lost, the current on the other two phases would have increased to around 5.5A, but that was still WAY below the pick-up point of the OL relay. So it held in until the fuse finally cleared.

Your motor winding insulation is likely compromised too by the way. It may still work for a while, but it's life has been cut short now. The heating effect on the winding insulation is INCREASED when the motor is single phased.
 

kwired

Electron manager
Location
NE Nebraska
Is the motor still good? Your setup description basically tells us the protection level is high enough and possibly non existent on L3, that it may as well have been totally non existent. Your fuse that blew possibly blew when motor finally was overheated enough that a ground fault or even line to line fault developed within the motor.
 
KWIRED - Unknown on the status of the motor. It has been removed and will be used for testing later in the week. The tech that investigated said that the phase to phase resistance readings were equal for all three, but he did not say what the reading is.

ALL - Thanks for all the info. We have reset all of the motor starters for 3.5A. You have been a big help in understanding how the OL works.

Todd
 
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