Arc Flash Mitigation on LV Switchboard

[rockman7892]

I was interested from hearing from some of the Arc Flash experts here what their
thoughts are for installing an arc flash relay (current and light sensing) for arc flash mitigation on a LV switchboard? Is it practical to install CT’s in incoming section with remote trip to upstream breaker? I would assume these CT’s would need to be installed on the line side of the main breaker on the incoming cables ahead of the breaker in order to mitigate hazard on line side of main breaker.

With switchboard AF mitigation the solution I’ve seen most often is to implement a maintenance mode in upstream breaker to reduce AF at downstream switchboard.

 

[thermionic1]

I'm starting a project shortly where AF protection is being added, although this is 50 year old 13.2kV gear.

We are using SEL-751 relays with AF option. We are then using SEL mirror bits via serial to fiber converters to trip the upstream feeder, in a separate building. SEL also has a SEL-851 coming soon that has AF built in. Obviously, your 480V breakers will require a shunt trip system. Their are loop sensors for the main bus and point sensors in the front racking shutter area. We have an issue in one line up where the CT's are on the load side, to the feeder sensors connect to the main relay.

I recently did a brand new 27kV vacuum project where the AF (SEL-751) was factory installed. This had both the main bus loop as well as point sensors in each of the rear cable terminations and behind the shutters in the front. This was a data center, "Cadillac" line up that also contained Partial Discharge monitoring, mandatory remote racking and an arc resistant design. Just an interesting side note, but the facility has 10MW diesel, Solar, Wind, (4) 138kV feeds and this latest site is in an abandoned quarry where I understand the pit will be flooded and used as a heat sink for geothermal.

If you have not checked, SEL has some app guides and installation notes available. 1/3 Swgr sections we are dealing with have air magnetic breakers, which makes things a bit trickier than the others which are vacuum. SEL has a paper on this as well.

Installing new CT's may present a problem with space in the 480V cubicle. Many times even the factory will have a CT on A&C phase of line side and B Phase on load side due to space limitations. I've seen CT's mounted in the cable entrance area, but that generally requires disconnecting and rerouting the the incoming cables through the CT, which depending on the number of cables or bus bar requires more installation time.

I believe ABB also makes a dedicated AF unit, but at least in the US no-one will complain about a SEL installation.

 

[dpc]

First issue: There must be a breaker upstream (separate from the switchboard) that can be tripped.

Second issue: On a close-coupled substation, installation of CTs ahead of the main breaker can be challenging. If cable-connected, it should be feasible. Bus side CTs in the main breaker cubicle itself will not be fully reliable in detecting faults in the main breaker section due to risk of arc propagation.

The light detection approach is fundamentally different then tripping on overcurrent, although current sensing is still necessary. Light sensors need to be installed inside the equipment being protected, including all sections and compartments. It will likely require a significant outage. But again, there must an upstream breaker to trip.

Cheers,

Dave

 

[unclebob]

That line side will need to be separeted from the load side with some Bakelite/fiber barrier.

 

[rockman7892]

Unclebob

Your comment was what my main concern was with this application. However as dpc points out if the switchboard is cable connected and CT’s can be placed on line side of main breaker then I’d think that this would mitigate arc flash on both line and load side of main breaker thus not requiring separation of main.

As I understood it, the separation you mention is only requires when sensing fault on load side of main breaker thus leaving line side exposed to high incident energy faults?

 

[unclebob]

Putting a barrier will isolate the main breaker from arc propagation coming from the load side.
That way, the breaker will mitigate the AF on the load side, while the line side CT's (with their dedicated feeder breaker) will lower the incident energy on the line side.
The barrier assure the main breaker will still operate if an AF occurs close to it.
We use double labels stickers when dealing with main breakers, for line-side and load-side.
But if no separation exist, no one can tell what is going to happen if the main breaker is affected by the AF, so we can't assume it will open and clear the fault.
This case is like having a switchgear/board without a main, with a significant larger AF category.

 

[wbd]

Unless the barrier is a tested configuration, I don't believe you can take credit for it isolating an arcing fault. Unless it is specifically arc rated and testing equipment, I assume the arc will jump to the line side of the breaker and the upstream remote protective device will clear the fault.
I would not want to be involved in a liability case where a non-tested barrier configuration was installed and I took credit for it in my analysis. I think that would be a losing situation on my part.

 

[unclebob]

No one wants to be liable these days. Beginning with the utilities that won't give you the protection settings, because the may change without notice, or give you a stupid range of s-c contribution going from 1 to 500 kA.

 

[che12345]

Dear Mr rockman7892
R. ".... what their thoughts are for installing an arc flash relay (current and light sensing) for arc flash mitigation on a LV switchboard? Is it practical to install CT’s in incoming section with remote trip to upstream breaker?..."
C1. Some fifty years ago; I presented in Singapore, a mock-p (full scale) 400V main incoming board with ACB installed with the first version of the Arc-flash relay; a product from a reputable manufacturer in Europe. Note: the {first version} was with [light sensing] only. When triggered by a [camera flash], the ACB tripped <100ms. To avoid inadvertent tripping, a warning label shall be prominently displayed at the entrance of the switch room "Prohibition of use of camera flash". The subsequent versions includes [current + light sensing].
C2. coverage of protection:
a) incoming line side of the ACB, CTs would have to be installed at the line side. Trip the up-steam MV breaker.
b) out-going load side of the ACB, CTs would have to be installed at the load side. Trip the ACB.
C3. Is it usefulness?
a) It is acknowledged that tripping at [<100ms] would reduce the damage to the property/board/equipment and risk to the operation personal.
b) It is confirmed that the (time-delayed over-current tripping time) of [ACB, MCCB or fuse] would NOT operate at <100ms with arc-flash current that is lower than their rated current.
c} It is defiantly useful. It is a small premium to pay for the insurance.
C4. What is the trend/acceptance in the industry?
a) This concept (light + current sensing) have been around for about seventy years.
b) its usage is far far below than the expectation.
c) I have NOT been informed of any bad report. It could be that very small number installed.
d) It is neither widely included in the tender documents, nor mandatary in the Standards or the Rule/Regulations in the marine boards.
Che Kuan Yau (Singapore)

 

[rockman7892]

Thanks che12345

Does anyone have an idea on response time for this AF relay application from initiation of fault to clearing of upstream breaker in LV application?

From the SEL literature I read it stated that the combination of current a light detection could sense an arc fault in a 1/4 cycle (less than 5 cycles). Assuming there is no lockout relay and there is a direct trip output from the relay to the shunt trip coil on an upstream power breaker how much time is usually considered for overall clearing time for Incident Energy calcs?

I've seen the light sensing (point sensors etc...) in MV and LV Switchgear applications but have never seen in a LV Switchboard application. Does anyone have familiarity with application in LV Switchboard or know if there is any industry or manufacturer guidance?

Thanks

 

[thermionic1]

The SEL-849 has AF protection and is designed to operate in LV environments. Why not give them a call?

 

[che12345]

Dear Mr rockman7892
R. "...Does anyone have an idea on response time for this AF relay application from initiation of fault to clearing of upstream breaker in LV application?.."
C1. I worked in Singapore, employed by then ASEA and later ABB for more than thirty years. The first version was with light sensing only. It was TVOA by ASEA. The later versions by ABB are with light and current sensing.
C2. I understand that the reaction time (from the time the light is detected to the output signal) is approx 2ms.
C3. The total time (from the time the light is detected to the breaker fully open ) is dependent on the ACB or MV breaker opening time; in general <70ms. Total time generally < 100ms.
C4. I have neither seen nor being informed of any LV incoming/distribution or MCC installed with it; on land or on marine boards.
Che Kuan Yau (Singapore)