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Old 08-03-2013, 10:47 AM   #1
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Default Any Ideas?

This could get long so bare with me.......

400A 3 phase wye


One of our stores we service has blown 6 - 400A time delayed fuses on phase B in their fused disconnect over the past 2 weeks. So lets look at how this has played out thus far.

Day1 - 11pm: Store only has partial power and has been that way since 1pm. They first called the Poco and waited 8 hours for them to tell them that it wasn't their problem. We find a blown 400A phase B fuse in the fused disconnect. Bad storms so we replace and everything seems fine.

Day2 - Same fuse blows again at the same time around 1pm. Now this is where it starts to get interesting. This store has a 112.5kva transformer hooked up to boost voltage because they have had a problem with low voltage for years. We decide to open up the transformer for ****s and giggles and find that the lug terminal on coil B is badly burnt and the wire is basically burnt off. We wire brush it and make neccessary repairs. Everything seems fine. We replace fuse and fire the store back up.

Day3 - Fuse blows again around the same time. This time we come loaded with our Fluke 434 and decide we are going to let it hooked up in the CT cabinet for a few days to monitor the 3 phase service and see what kind of loads and voltages we are getting. When we initially hook it up and replace the fuse we find that with none of the stores cooking equipment on that we are getting about 300A on phase B and close to that on the other phases as well. Note this is a 400A service so we make the conclusion that it has to be overloaded when the store is running all of their ovens and such. We are also seeing very low voltage at this point. We make a call to the Poco and have a trouble guy come out. We explain to him that their incoming voltage is low. We were seeing as low as 105 volts at peak loads. We figure if we are going to have to do a service upgrade to 600A then we want the poco to fix their low voltage problem at the same time. Note that the transformer pots are all the way down the side of the street and actually cross the street. From the pots over to a service head on the side of the store they ran a single set of 3phase 4/0 when they should have run parallels. The poco explains to us that in order to do any serious changes like running parallels or bringing 3 phase primary over across the street closer to our store that it takes 6 weeks of planning. Well our store doesn't want to wait 6 weeks they need a quick fix. They reccomend that they upgrade their pots from 25's to 50's and tell us that it should help our voltage problems. The poco comes out over the weekend and upgrades the pots to 50's. It has helped our voltage somewhat but we are still seeing as low as 109 volts at peak loads. This is where we decide to meet with the power man from the poco to come up with a game plan on how to fix this issue altogether. He agrees that they need to run parallel 4/0 from their pots over to our store. We decide that best would be for us to set a pole out back and mount our CT cabinet right next to it. So the poco will bring their paralleled 4/0 over to our pole. We will run it down the pole underground over to the new CT cabinet. This will allow us to get rid of the service head and pipe that are running along side the stores outside wall and around back about an extra 75 feet. We figure the parallel 4/0 should cut our resistance in half. So this is where it gets tricky.

In the mean time the store continues to blow fuses however they are blowing 400A time delayed fuses at low loads. Our meter showed max 200A around the time the last fuse blew and thats where we are scratching our heads. We shot our thermal imager on the fused disconnect just to see and we are reading about 250-300 degrees on the load side of phase b in the discconect. I asked my dad if he opened up the transformer to see if it was heating up on phase b coil in their as well. So now were thinking maybe the transformer is starting to go bad because of how badly it was burnt. Wondering if anyone has had any experience like this or has any ideas for us? Sorry if this was a long message and if I left anything out don't hesitate to ask. Im young only 26 and still have so much to learn.

Thanks.
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Old 08-03-2013, 11:01 AM   #2
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Not reading all that.

Did you perform a fall of potential or IR

This ain't magic.
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Old 08-03-2013, 11:12 AM   #3
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Not reading all that.

Did you perform a fall of potential or IR

This ain't magic.
250-300 degrees the TI showed on the load side of phase b in the disconnect. Didn't shoot the transformer to see if phase b coil was heating up. regardless once the poco fixes their low voltage problem then we will no longer need the transformer anyways. The TI showed phase B glowing cherry red on the load side of the disconnect.

Last edited by SS&E1852; 08-03-2013 at 11:15 AM. Reason: Added info
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Old 08-03-2013, 11:25 AM   #4
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Originally Posted by SS&E1852 View Post
250-300 degrees the TI showed on the load side of phase b in the disconnect. Didn't shoot the transformer to see if phase b coil was heating up. regardless once the poco fixes their low voltage problem then we will no longer need the transformer anyways. The TI showed phase B glowing cherry red on the load side of the disconnect.
At only 200 amps?

Bad connection on the load side terminal?
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Old 08-03-2013, 11:27 AM   #5
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At only 200 amps?

Bad connection on the load side terminal?
That's what we thought. We shut the store back down and tried tightening the connection on phase b and it wouldn't budge.
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Old 08-03-2013, 11:28 AM   #6
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Originally Posted by SS&E1852 View Post
250-300 degrees the TI showed on the load side of phase b in the disconnect. Didn't shoot the transformer to see if phase b coil was heating up. regardless once the poco fixes their low voltage problem then we will no longer need the transformer anyways. The TI showed phase B glowing cherry red on the load side of the disconnect.
is this the temperature at the fuse? If so there is you problem,

What was the voltage drop across the fuse?
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Old 08-03-2013, 11:34 AM   #7
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is this the temperature at the fuse? If so there is you problem,

What was the voltage drop across the fuse?
No voltage drop across the fuse. The transformer however is fed from the load side of that disconnect which is glowing cherry red. I attached a picture of when the wire heated up so much that it actually burnt off. Note these fuses seeming to be blowing on very hot days.

Thanks
Attached Thumbnails
Any Ideas?-image.jpg  

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Old 08-03-2013, 05:42 PM   #8
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No voltage drop across the fuse.
Thanks
Every part of an electrical distribution system has voltage drop, high resistance results in a high voltage drop across a connection and that will drive the temperature up. One function of that fuse you are talking about is a thermal device and that could be your problem. The high resistance connection at the transformer may be causing you some issues but I doubt it is resulting in a thermal issue with the fuse.

Perform a FOP (Fall of Potential) test on the Fused Saftey Switch.
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Old 08-03-2013, 05:48 PM   #9
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I wrote this a while ago.

We are often called by electrical contractors to investigate why fuses are blowing or CBs are tripping. Many times the cause is a high resistance connections resulting in sufficient heat to effect the thermal element in the fuse or CB.

A simple method to isolate the high resistance connection and thus the source of the heat is the Fall of Potential Test Method, commonly referenced to as the FOP test. To perform this test, one simply needs a multimeter with a millivolt scale, and an amp clamp.

There needs to be a load on the device to be tested, preferably a balanced load or close to balanced load. In the case of a fused safety switch (FSS). One would measure current across all three phases, then measure from line to load of one pole/phase of the conductor strands (if exposed) for each pole of the FSS. If one phase has a higher that average millivolt measurement (actually the voltage drop across the device under test). Your next measurement would be from line conductor to line of the fuse, if all readings are close to equal move to the next components of the FSS, in this manner you an isolate the high resistance connection.

With an arranged outage repairs can be implemented and a repair FOP measurement taken to verify repairs.

Our thermographers perform this test as part of their IR Scan to isolate to high resistance issue. As sometimes it is not possible to determine from a picture if the issue is a CB connection to the bus or the CB. Additionally it is not feasible to use a DLRO (Digital Low Resistance Ohm Meter)/ Micro ohm-meter to take measurements on small CBs and FSS due to contact point spacing of the test instruments, so our technicians take pre-repair and post-repair measurement s to verify repairs.


An example we IRd a 200 amp CB this weekend with 155 amps per phase (average), millivolt readings were 38mv, 91mv and 42 mv. The readings were taken from the bus stabs of the CB, negating any possible issue with the CB to bus connection or conductor termination connector to CB connection. B phase had an issue, when we replace the CB we will do further testing and open the CB to see if visual thermal damage has started.

This test can be performed on single pole CB, or any 3-pole devices, we have used this on 4000 amp bolted pressure switches.

As with any testing of exposed energized parts, all safety cautions must be observed, wearing of PPE, isolating the area to be worked in. One issue we have had over the years is customers taking FLASH photography as we are taking measurements. We no longer permit customers to take photos, without prior notice. This minimizes heart attacks
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Old 08-03-2013, 06:09 PM   #10
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Originally Posted by brian john View Post
I wrote this a while ago.

We are often called by electrical contractors to investigate why fuses are blowing or CBs are tripping. Many times the cause is a high resistance connections resulting in sufficient heat to effect the thermal element in the fuse or CB.

A simple method to isolate the high resistance connection and thus the source of the heat is the Fall of Potential Test Method, commonly referenced to as the FOP test. To perform this test, one simply needs a multimeter with a millivolt scale, and an amp clamp.

There needs to be a load on the device to be tested, preferably a balanced load or close to balanced load. In the case of a fused safety switch (FSS). One would measure current across all three phases, then measure from line to load of one pole/phase of the conductor strands (if exposed) for each pole of the FSS. If one phase has a higher that average millivolt measurement (actually the voltage drop across the device under test). Your next measurement would be from line conductor to line of the fuse, if all readings are close to equal move to the next components of the FSS, in this manner you an isolate the high resistance connection.

With an arranged outage repairs can be implemented and a repair FOP measurement taken to verify repairs.

Our thermographers perform this test as part of their IR Scan to isolate to high resistance issue. As sometimes it is not possible to determine from a picture if the issue is a CB connection to the bus or the CB. Additionally it is not feasible to use a DLRO (Digital Low Resistance Ohm Meter)/ Micro ohm-meter to take measurements on small CBs and FSS due to contact point spacing of the test instruments, so our technicians take pre-repair and post-repair measurement s to verify repairs.


An example we IRd a 200 amp CB this weekend with 155 amps per phase (average), millivolt readings were 38mv, 91mv and 42 mv. The readings were taken from the bus stabs of the CB, negating any possible issue with the CB to bus connection or conductor termination connector to CB connection. B phase had an issue, when we replace the CB we will do further testing and open the CB to see if visual thermal damage has started.

This test can be performed on single pole CB, or any 3-pole devices, we have used this on 4000 amp bolted pressure switches.

As with any testing of exposed energized parts, all safety cautions must be observed, wearing of PPE, isolating the area to be worked in. One issue we have had over the years is customers taking FLASH photography as we are taking measurements. We no longer permit customers to take photos, without prior notice. This minimizes heart attacks
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Thanks Brian. You really know your ****. Will give this a try this week.
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Old 08-03-2013, 06:17 PM   #11
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Thanks Brian. You really know your ****. Will give this a try this week.
43 years doing this and thanks to many electricians much smarter than me, I have glombed a bit of knowledge
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Old 08-03-2013, 06:25 PM   #12
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Moved.
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