Trip coordination

[scott minter]

Hello I encountered a problem at our facility, the failure of a cooling tower recirculation motor

tripped the MCC bucket (400a) that fed the panel, which held the breaker (50a) that fed the

VFD to the motor. I found both trip curves on line and compared to each other and found

they cross. So i moved the 400a breaker up from instantaneously to 100ms, which on paper

would seem to fix the problem. My question is : does that mean I

have to now take 240.87 into consideration? Thanks scott

 

[JoeStillman]

If you have disabled instantaneous on the 400A, then yes, 240.87 applies.

But is that really necessary? Was the instantaneous cranked up to maximum on the 400A? They always ship with the minimum setpoint on the dial and nobody ever seems to adjust them until after they trip one time. It may be possible to set the instantaneous on the 400 higher than the availabel SC current on the load ends of the branches. Then you'll be truly coordinated. I would bet that the SC a lot lower on the end of a 50A circuit than at the 400A main.

I'm surprised the 50A and 400A couldn't be coordinated.

 

[scott minter]

reply

reply

Hi, thank you for your reply,

I did not disable the instantaneous trip, I reset it from instantaneous to 100 milisecond delay

that moved my curve and will allow the 50a to trip first. I am under the impression that is

how you coordinate a circuit. I am working on all the other loads to make sure they cant

do what the 50a did. If i do not have to move it anymore (100ms) I will leave it set the way it is.

But will I need to now take 240.87 into consideration? Thanks scott

 

[jim dungar]

But will I need to now take 240.87 into consideration? Thanks scott

What type of device do you have?

In general if it is a molded case breaker, especially one built to UL489, there is nothing you can do in the field that can truly disable Instantaneous protection.
Typically, 240.87 is applicable to devices like protective relays and power circuit breakers with replaceable trip units.
The next NEC will probably have changes to this section.

 

[scott minter]

reply

reply

68C4012G18 3071F HKDB 65K Elect Sealed Breaker-Reverse Feed 3 Pole 400A

HKDB3400FT32W

782113713076

CUTLER-HAMMER INC The problem is that the 400 Amp breaker was set to instantaneous on its time delay. With this setting, it was a horsace which breaker will trip first, the 50 amp breaker or the 400 amp breaker, for a fairly wide range of potential short-circuit fault currents. Do I need to concern myself with 240.87 now that I have made the change?

 

[Mike01]

Overlap / Coordination

Overlap / Coordination

Looks like this is a Molded Case Circuit Breaker [UL489] with an adjustable electronic trip unit [310] [LSI], it appears the instantaneous on this unit cannot be disabled due to the fact it is a MCCB as indicated below, however the breaker can be adjusted in its short time and instantaneous region [pushed farther to the right on the TCC] to allow for better coordination of downstream devices, but you would need to know what the available fault current is to "clip" the instantaneous portion of the curve at the fault current so you can see the "overlap" and where it exist so you can adjust accordingly. Also I do not know that the facility's stance on arc flash is but the greater the tripping time the potential for a greater arc flash hazard thus comes into the conversation selectivity vs. Arc-Flash.

 

[Jraef]

Just so you know, your failure would NOT be in the motor if there is a modern VFD between the motor and the circuit breaker. The VFD would isolate the motor fault current from the line, and in fact UL requires that the VFD now be listed as the SCPD for the motor connected to it. The breaker feeding the VFD is only going to see faults between the breaker and the VFD line terminals. In your case if it was the 400A breaker that tripped, it was tripping on a fault between that 400A breaker and the 50A breaker.

 

[scott minter]

Thank you, same question

Thank you, same question

Thank You very much for all the help. I am cognizant of the VFD's function. When the original issue

occurred the VFD did indeed catch the fault, to continue our hazardous and critical test I bypassed

the VFD and went stright to the 50a breaker. This was under direction of our control's engineer who

experianceing some VFD problems. We discovered the bad motor, removed, used spare and

got our test complete. The primary motor is back in and the 400a set off instantaneous to 100

millisecond delay. But will I need to now take 240.87 into consideration? Thanks scott

 

[jim dungar]

But will I need to now take 240.87 into consideration? Thanks scott

Not with your style breaker. The Instantaneous (the vertical) portion of the trip curve still exists, you have simply shifted its position.

 

[scott minter]

Now i understand

Now i understand

Thank you for all the replies.

This place is great.