The weakest link

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mahobbs

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Indianapolis, Indiana
We hade a problem this morning, and I have to explain it to administration, and I do not know how.

1. A 40 HP, 46.5 Amp, 460 Volt, 3 phase motor, went bad.
2. The 80 amp fuses in the Graham VSD did not blow.
3. The 150 Amp breaker in the distribution panel did not trip.
4. The 1600 Amp, 480 Volt, feeder breaker for the ATS (Normal Power) tripped, (ground fault flag pop out).
5. The EPSS started
6. The 1600 Amp, 480 Volt, feeder breaker for the ATS (Emergency Power) tripped (ground fault flag pop out).

The setting on the NORMAL power breaker
Current Setting 1.0X
Instantaneous Pickup 15X
Long Time Delay 2
Ground Fault Pickup .35S
Short Time Delay 5C
Ground Fault Delay MIN
Short Time Delay MIN

The setting on the EMERGENCY power breaker.
Current Setting 1.0X
Instantaneous Pickup 15X
Long Time Delay 2
Ground Fault Pickup .2X
Short Time Delay 5C
Ground Fault Delay MIN
Short Time Delay MIN

Why did the 80 Amp fuses not stop the ground fault (if that was the problem)?

I have always been told that when this happen, the weakest link will let go first.

Thank you for your help

Mike
 
most likely you had a ground fault. the breakers with the gf feature tripped before the downstream breakers had a chance to do so. ground fault protection is instantaneous, where as the downstream stuff is not.

the weakest link does not trip first. the fastest link does.
 
Is "Ground Fault Pickup .35S " = 0.35 seconds?
Do you know how long "Ground Fault Delay MIN" is in seconds?
 
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emergency system or optional standby ? (isn't emergency required to have coordination to prevent this exact thing ?)
 
In the 2005 Yes.

700.27 Coordination
Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.
 
would be nice now that you have a known faulty condition to be able to take the time (if possible) and play around with the fuses to see if you can at least recommend a set of fuses that works (when you give them the bad knews)
 
GF Setting

GF Setting

My advice is to set the GF trip of your highest-level breakers at 1200 amps - which is the maximum allowed by NEC. This will also be the maximum available setting on the breaker for GF trip. That will give the lower level phase devices a chance to operate.


.. also set the GF delay at MAX for the highest level breaker in the system.


JM
 
I don't see how a ground fault on the load side of a VFD would trip a line side ground fault protective device. The output of a VFD is, for all practical purposes, a SDS.
Don
 
The fuses would only blow with a ground fault if there was a direct short to ground, and even then the ground fault protection device should still trip first. However, I would think that the VSD would have faulted before anything else.
 
It looks like you need to have a coordination study performed to determine the correct setting for your desired level of operation. Although with only a single level of ground fault protection, it is rare that you will be able to fully coordinate with any device larger than 70A regardless of your settings.

It looks like your normal main breaker ground fault is set for: .35 x sensor = .35x1600 = 560A. With the delay set to minimum it is probably at .1 sec. Likewise your emergency breaker is .2x1600 = 320A which probably doesn't even coordinate with a 30A branch breaker.
 
don_resqcapt19 said:
I don't see how a ground fault on the load side of a VFD would trip a line side ground fault protective device. The output of a VFD is, for all practical purposes, a SDS.
Don
Click to expand...
While I'm certainly no expert in the area, I'd say a VFD isn't an SDS unless it has an isolating transformer at the input stage. The ones I have worked with did not, IIRC.
 
Smart,
While I'm certainly no expert in the area, I'd say a VFD isn't an SDS unless it has an isolating transformer at the input stage.
Click to expand...
Every one that I have worked with converts the incomming AC to DC and then converts it back to supply the motor at what ever frequency that is required. That is an SDS to me. A ground fault on the load side is not seen as a ground fault on the line side.
Don
 
don_resqcapt19 said:
Every one that I have worked with converts the incomming AC to DC and then converts it back to supply the motor at what ever frequency that is required. That is an SDS to me. A ground fault on the load side is not seen as a ground fault on the line side.
Don
Click to expand...

The key feature of a SDS is not the _conversion_, but instead _isolation_.

A transformer can be an SDS because there is no electrical connection between input and output. Without additional connections, there is no continuous conductive path from input to output.

In a VSD, you have a continuous conductive path through the input rectifier, then through the DC bus, and then through the output switching elements. Because of the nature of the switching devices, this path will continuously change, but it still exists.

Consider a grounded wye feeding a VSD, and short one of the output legs to ground. Imagine that phase A is most positive amongst the transformer legs, and the VSD output is switched to connect to the positive rail. Current will flow from phase A, through the rectifier, through the positive rail, through the output leg, to ground and thence back to the grounded terminal of the transformer.

-Jon
 
I have to disagree if the isolated output section of this gear had no effect on the input how would the electric meter know how many KW are being used.

Thats like saying if I short out the secondary of a DC power supply the transformer primary wouldn't be effected.

The motor drive is still hook to the primary systems.

It is hooked through triacs or double pole SCRs. if you short the output of a triac it no different from shorting a regular switch.

The only way you could get complete isolation is convert as Don said , but it would have to removed from the source with hard contacts and then rehooked when charge got low to recharge. :)
 
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ronaldrc said:
I have to disagree...
Click to expand...
Not sure exactly what you're disagreeing with, but we are saying the output of the OP's VFD is not isolated from its supply. Therefore it's output does not qualify as a separately derived system. Rather, it is a direct-connected switching supply, and ground fault current returns to the secondary of the line-side transformer. As such, the fastest-to-operate ground-fault-protective device under the fault condition presented will trip...
 
Smart $ said:
Not sure exactly what you're disagreeing with, but we are saying the output of the OP's VFD is not isolated from its supply. Therefore it's output does not qualify as a separately derived system. Rather, it is a direct-connected switching supply, and ground fault current returns to the secondary of the line-side transformer. As such, the fastest-to-operate ground-fault-protective device under the fault condition presented will trip...
Click to expand...


I don't know what you are referring to but I was in disagreement with Don
when he said the output would not effect the main Service or what ever breakers or fuse where upstream.

We all have disageements Sir.


I put my foot in my mouth a lot of times because I don't take time to think it out.

Don has corrected me before and rightly so.

I do have a right to input my opinion I hope?

Edited to insert a statement

And a SDS has nothing to with it, even if it was a supplied with transformer with a shielded secondary
if the secondary becomes shorted it could still smoke the primary.
 
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ronaldrc said:
I don't know what you are referring to but I was in disagreement with Don...
Click to expand...
Well that certainly helps, since your post was listed as a response to Jon's (aka winnie) :smile:

We all have disageements Sir.
Click to expand...
Yes we do.

I do have a right to input my opinion I hope?
Click to expand...
Yes you do... and I'll be the last person to tell you that you don't.

And a SDS has nothing to with it, even if it was a supplied with transformer with a shielded secondary
if the secondary becomes shorted it could still smoke the primary.
Click to expand...
Yes. But the point was the matter of where the ground fault current returns to on the output side of the vfd. It does not return only to the vfd because it is not isolated from the line side.
 
ronaldrc said:
I have to disagree if the isolated output section of this gear had no effect on the input how would the electric meter know how many KW are being used.
Click to expand...

I know that you've made later posts, but I was going to address this point.

The concept that Don was applying is that an SDS will not trip an upstream ground fault detector. I happen to agree with Don on this point.

Clearly, doing something like shorting the secondary of a transformer will have an effect on the primary circuit. Lots of current flowing on the secondary means lots of current flowing on the primary. But any of the current flowing on the primary side will flow on the normal feeder conductors. There will be no ground fault current on the primary side; simply an excess of current flow on the normal conductors.

The error that Smart$ and I pointed out was that a VFD was probably not an SDS. A VFD can be designed as an SDS, by including some sort of internal transformer that provides galvanic isolation, but I don't know if these even exist; they certainly are not common.

-Jon
 
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