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 Post subject: Arc-hazard on MCCB Line-Side or Load-Side
PostPosted: Tue Jul 19, 2011 1:19 pm 
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Assume the construction of an MCC provides compartments such that an arc in one compartment can not propagate to another compartment.
eg. Tests have been done to ensure an arc in an MCC module will not propagate beyond the MCC module, all barriers will remain effective.
The MCCB line-side terminals connect directly to a socket which connects to the vertical bus bar in the compartment behind the MCC modules. ie. the MCCB line-side contacts are not accessible from inside the MCC module.

Question 1.
Will the arc-hazard label on the MCC module door include the arc-flash hazard for an arc on the load-side of the MCCB, or on the line-side terminals of the MCCB ?
Must the possibility that an arcing fault on the MCCB load-side terminals damages the MCCB such that the arc can propagate to the MCCB line-side terminals be considered ?

Question 2.
Assume that the arc-flash boundary due to an arcing fault on the line-side terminals of the Incomer ACB extends beyond the MCC. Does this mean that the only applicable arc-flash hazard label is that for an arcing fault on the line-side of the Incoming ACB ?

Question 3.
If the MCC has been tested to ensure an arc in any compartment of the MCC will not propagate outside the compartment and all barriers remain effective ; are arc-flash hazard lables only relevant for when MCC compartment doors are open for testing ?


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PostPosted: Wed Jul 20, 2011 2:34 pm 
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mwjnewman wrote:
Assume the construction of an MCC provides compartments such that an arc in one compartment can not propagate to another compartment.
eg. Tests have been done to ensure an arc in an MCC module will not propagate beyond the MCC module, all barriers will remain effective.
The MCCB line-side terminals connect directly to a socket which connects to the vertical bus bar in the compartment behind the MCC modules. ie. the MCCB line-side contacts are not accessible from inside the MCC module.

Question 1.
Will the arc-hazard label on the MCC module door include the arc-flash hazard for an arc on the load-side of the MCCB, or on the line-side terminals of the MCCB ?
Must the possibility that an arcing fault on the MCCB load-side terminals damages the MCCB such that the arc can propagate to the MCCB line-side terminals be considered ?

Question 2.
Assume that the arc-flash boundary due to an arcing fault on the line-side terminals of the Incomer ACB extends beyond the MCC. Does this mean that the only applicable arc-flash hazard label is that for an arcing fault on the line-side of the Incoming ACB ?

Question 3.
If the MCC has been tested to ensure an arc in any compartment of the MCC will not propagate outside the compartment and all barriers remain effective ; are arc-flash hazard lables only relevant for when MCC compartment doors are open for testing ?


Question 1 - is it arc rated? I would base it on the line side. The fireball has a way of getting through small cracks according to videos out there. Was it tested so the doors don't blow off? Lots of pressure and it has to go somewhere.

Question 2 - I think this one depends on the design. If you are saying the MCC is built in such a way that the arc flash can not propogate, then probably use the main (incomer?) as the limiting device for the branches.

Question 3 - OK so it sounds like this might be arc resistant equipment. Tested to which standard? There was a question here the other day about lableing arc resistant equipment. I think the suggestion was label it based on the calculated incident energy but then have a note about category 0 if the doors are closed.


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PostPosted: Wed Jul 20, 2011 3:28 pm 
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J. Wells wrote:
Question 1 - is it arc rated? I would base it on the line side. The fireball has a way of getting through small cracks according to videos out there. Was it tested so the doors don't blow off? Lots of pressure and it has to go somewhere.

Question 2 - I think this one depends on the design. If you are saying the MCC is built in such a way that the arc flash can not propogate, then probably use the main (incomer?) as the limiting device for the branches.

Question 3 - OK so it sounds like this might be arc resistant equipment. Tested to which standard? There was a question here the other day about lableing arc resistant equipment. I think the suggestion was label it based on the calculated incident energy but then have a note about category 0 if the doors are closed.


The arc is vented and contained within the bus chamber/compartment thru a flap. When an arc initiated within a starter cell, it will be vented to the bus chamber. The test is conducted by a third party and it is up to the inspector to decide on the location of where the arc shall be initiated.

I have attached a photo of how the circuit breaker is connected. This is how we've been designing our MCC. As you can see, there are no exposed conductor on the line side of the MCCB. It is fully covered with shroud and connected via plugs to the main bus. Another thing to note is the busbars are insulated as well, so again thats reducing the probability of an arc occuring. May I add that I am not concern with the load side as that is a very straight forward answer but it is the line side which doesnt make sense to me.

I am currently waiting for Schneider's response regarding the protection of the shrouding. Because the MCC manufacturer has confirmed that any arc fault will not propagate into another chamber/compartment, it is clear that the line connection of the MCCB is what concerns me.

The reason why I am keen to rate the cell according to the load is because the operators can wear a appropriate category PPE to test for dead when the door is open and is able to remove the PPE when confirm there are no live parts in the cell. Of course, if any work needs to be done on the MCCB (e.g. replacing MCCB), the upstream incomer (ACB) will need to be open. If an arc cannot be initiated at the red circled area, the cell will be safe to work in after testing for dead.

The MCC is tested to AS/NZS 3439.1 Annex ZD (Australian Standard) with an additional special test of the incomer and the buschamber in part of Annex ZD6.

Heres a general summary of the Annex

Quote:
Tests shall cover representative types of compartments with respect to location in the
ASSEMBLY, e.g. end compartments, top compartments, bottom compartments and
compartments surrounded on all sides by other compartments, where such configurations
exist.
The ASSEMBLY shall be connected to a source capable of delivering a short-circuit current at
the terminals of the incoming circuit of the ASSEMBLY equal to the rated prospective,
conditional or fused short-circuit current of the ASSEMBLY, and with a power factor in
accordance with Clause 7.5.3. The recovery voltage shall be at least equal to but not greater
than 110% of the rated operational voltage of the ASSEMBLY.
The equipment in all compartments adjacent to the compartment under test shall be energized
to detect spreading of the arc to adjacent compartments. Any outgoing cables normally within
the compartment under test shall be installed as in service with any glands, or similar
equipment.
Where the ASSEMBLY being tested incorporates protective devices intended to limit the
duration or magnitude of the arcing current, these devices shall be set up as in normal service
with any adjustable elements set to the maximum setting recommended by the ASSEMBLY
manufacturer. These settings shall be recorded in the test report, together with details of the
protective devices. The test supply shall be maintained at the terminals of the incoming unit
for a time at least equal to the time for the protective device to interrupt the test current plus
approximately 10 cycles.
The arcing test shall be performed at least once on each compartment. The test may be
repeated if, in the opinion of the testing authority, there is any doubt regarding the validity of
the test, e.g. any doubt that—
(a) an arc had been established;
(b) the protective device had operated;
(c) the location of arc initiation is valid for the arrangement of equipment within the
compartment; or
(d) the arc may transfer to the line-side of the protective device.
The compartment and the equipment in it may be repaired or replaced before each test. A
new unit may be used for each test if requested by the manufacturer. Degradation of
insulation due to carbonization or moderate erosion of metal parts is not necessarily
considered to render a unit unsuitable for a further test.


My understanding is that you can only walk around in a category 0 PPE if the enclosure is arc rated or contained (meaning it is not possible for the door to be blown). Else, the required PPE or distance shall be maintained.

Because all our designs are certified arc fault contained, operators can walk around in their normal PPE turning on and off an isolator and should only wear the required PPE when testing for dead (assuming only load side fault is possible). Of course this is what we're trying to archieve. It is a shame that we put in all the effort in the design but theres nothing in the standards which says we can rate the enclosure to a lower PPE rating due to the additional protection.


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PostPosted: Mon Jul 25, 2011 7:35 am 
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Since AS3439.1 is the standard for Arc Fault Containment and is subject to testing by “Testing and Certification Australia”, the equipment is designed to protect the worker in the event of an arc flash as long as the enclosure / cover is properly closed. Similar to the U.S. ANSI rated arc resistant switchgear and MCCs

The design in the photo with the line side insulated looks more like compliance with Australian requirements of AS/NZS 3000 2.5.5 which states:

“protection against arcing fault currents while the equipment is in service or is undergoing maintenance, shall be provided for heavy current switchboards (800A or more)”

The actual language in 2.5.5.2 states: “Reduction of the [i]probability of the initiation of a switchboard internal arcing fault”[/i] then it lists the methods such as insulating the bus, internal separation, shrouding etc.

The word “probability” to me means it could still happen. However you are correct, AS design does appear safer than the US open exposed design. I had the opportunity to spend some time in Australia earlier this year and was very impressed with the equipment designs.

I would be interested to hear if the manufacturer will state that the line side shrouding will “eliminate” the possibility of an arc flash allowing you to remove PPE while the door is open and the line side is energized. (my bet is they will say no)

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PostPosted: Mon Jul 25, 2011 8:27 am 
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brainfiller wrote:
I would be interested to hear if the manufacturer will state that the line side shrouding will “eliminate” the possibility of an arc flash allowing you to remove PPE while the door is open and the line side is energized. (my bet is they will say no)


I'm not sure why not? Looking at the picture above, it appears all the energized parts, on the line side of the breaker, are fully covered. I agree if the breaker was “on” and the components within the bucket energized then that’s a different story.


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PostPosted: Mon Jul 25, 2011 10:27 am 
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SCGEng1 wrote:
I'm not sure why not? Looking at the picture above, it appears all the energized parts, on the line side of the breaker, are fully covered. I agree if the breaker was “on” and the components within the bucket energized then that’s a different story.


I hope you are correct. I am just thinking of similar conversations about plexiglass etc. It protects against contact that could initiate an arc flash but not against arc flash itself. I think this is where "reasonable risk" should play a large role and I don't see a major risk here. We'll see if thehe manufacturer does. Like I said, I hope you are right :)

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PostPosted: Mon Jul 25, 2011 2:45 pm 
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brainfiller wrote:
I hope you are correct. I am just thinking of similar conversations about plexiglass etc. It protects against contact that could initiate an arc flash but not against arc flash itself. I think this is where "reasonable risk" should play a large role and I don't see a major risk here. We'll see if thehe manufacturer does. Like I said, I hope you are right :)


We're still waiting for Schneider to comment on that bit but as far as MCC manufacturing is concern, they have already confirm the insulated busbar and bus plug arrangement will prevent the arc from propagating.

We just went to one of our supplier's factory yesterday and I was very impressed by the fully insulated busbar. They actually have custom insulation/covers for all the buses.

With that kind of set up, I dare say should an arc initiate, it will only be possible on the load side of the cell's MCCB. I cannot see how an arc can occur with the amount of insulation and covers these guys put in.


SCGEng1 wrote:
I'm not sure why not? Looking at the picture above, it appears all the energized parts, on the line side of the breaker, are fully covered. I agree if the breaker was “on” and the components within the bucket energized then that’s a different story.


I'm hoping for that too.

I reckon an inspector trained in this area should be able to perform checks and tests to certify a product to be arc fault proof.

Those videos on arc flash are most likely to be caused by poor installation or poorly constructed boards. To compare that to what we have is just ridiculous.


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PostPosted: Thu Nov 28, 2013 12:42 am 
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An older thread, but some matters of interest.
In relation to point 2, (and in part point 3) the incomer zone to low voltage MCCs is typically protected from the high voltage side of a distribution transformer. This protection may be either high voltage fuses or a circuit breaker. For the case of an arcing fault in this zone which includes the line side isolating contacts of rackable ACBs, the fault level may be around 30% of the bolted fault level and normally less than the instantaneous over-current setting of the HV breaker. Typically this is due to the protection needing to allow for transformer inrush currents & large motor starting currents (250kW or greater). In such case, even with extremely inverse IDMT characteristics for the over-current protection, both HV fuses and over-current released circuit breakers will typically take at least two seconds to clear this fault. Appendix B of IEEE 1584 suggests that for personnel incident energy exposure levels, it is unlikely that a person would remain longer than this time, so we generally limit our calculations to this time. This 2s of course, exceeds the internal arc fault containment type test time of around 300ms, so there is no assurance that an arc will be contained, even with techniques to promote self-extinction of the arc. However, the incident energy levels that result for many installations exceed 40 cal/cm2 and we are advised to isolate upstream at the HV device prior to operating these devices. Unfortunately, I have seen many assessments that have either assumed that IAC has dealt with the issue, or more commonly this key area has failed to be recognised as it requires a dummy bus to be inserted in your power system modelling software in order for the calculations to be performed.
In resolving the issue of long time with overcurrent protection, we therefore need a faster protection scheme if we wish to be able to utilise this device. In the case where upstream HV circuit breakers are used, then we can trip via some other detection method & arc flash detection is often used, enabling clearance times in the order of 80ms. (for us in Australia on 50Hz– 3 cycle breaker + relay time). However if fuses are used upstream, then we have nowhere to connect our arc flash relay & we need other approaches. Arc quenching devices attached to the incomer zone can provide a solution here.
Whilst insulation can be applied to the incoming conductors up to the ACB carriage in an attempt to provide an arc free zone, the ACB isolating contacts remain a location where arcing faults occur particularly during racking. A case of this is apparent in a frequently aired CCTV video of racking in a switchroom. In my experience I am aware of various incidents on both HV breakers including fully encapsulated bus & ACBs associated with the isolating contacts.
Hence, I am also interested in approaches to this problem.


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PostPosted: Fri Nov 29, 2013 7:21 am 
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I am aware of literature pointing to the idea that there should be a lower time-based cutoff in terms of development of an arc flash or blast but 300 ms sounds extremely long. Perhaps the easiest way to fix your dilema in terms of space is to use bushing ct's around the lv legs of the transformer feeding a relay operating a breaker on the hv side. This is sometimes called a "virtual breaker". The resulting arrangement trips the hv breaker due to faults on either the hv or lv side. Coordination is essentially unaffected since there are no branches in the distribution.


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PostPosted: Tue Dec 10, 2013 7:10 pm 
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Thank you Paul, I have seen this scheme drawn but not implemented. In my experience, arc flash relays have been more commonly used, which provide faster clearance. However, I am interested in what approaches have been being used commonly to the issue, particularly where the transformer has been fed by a set of HV fuses.


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