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 Post subject: Typical Settings for Switchgear Maintenance Switches
PostPosted: Mon Jul 29, 2019 8:38 am 
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Joined: Thu Jan 10, 2008 8:49 pm
Posts: 499
Location: New England
For those of you that have added 'Maintenance Switches' to your 480V switchgear, how are you determining the trip setting. I'd like to keep it as low as possible, so, I'm thinking something like 'inrush for largest motor plus 1x Long Time. This would result in something like 1.5 to 2.5 of the long time setting.
Don't think I need to worry about two or three motors starting simultaneously, as that isn't common for my install. Wonder what rule of thumb others are using.


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 Post subject: Re: Typical Settings for Switchgear Maintenance Switches
PostPosted: Tue Jul 30, 2019 10:09 am 
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Joined: Mon Jan 18, 2010 11:35 am
Posts: 524
Location: Wisconsin
I believe to meet the NEC 240.6 and 240.87 they need to be below the available arcing fault current.

Most of my customers have asked for settings that provide an incident energy level below their 'daily wear' PPE.


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 Post subject: Re: Typical Settings for Switchgear Maintenance Switches
PostPosted: Mon Aug 05, 2019 9:22 am 
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Joined: Tue Oct 26, 2010 5:00 pm
Posts: 191
Location: Maple Valley, WA.
We always set it to the lowest setting possible. So far, we have not experienced breakers tripping when a motor starts. As far as the Long Time, most Maintenance Switches do not have a long time settings. Most have just a pick up and short time/instantaneous settings. Most are like a Instantaneous Function with adjustable time delay. We set both the pick and time delay at minimum.

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 Post subject: Re: Typical Settings for Switchgear Maintenance Switches
PostPosted: Mon Aug 05, 2019 10:15 am 
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Joined: Thu Jan 10, 2008 8:49 pm
Posts: 499
Location: New England
Thanks all, but let me explain further.

Yes, Maintenance Switch is an Instantaneous trip setting. I'm going to say with no time delay.
So I have 1200 Amp Switchgear breaker feeding an MCC sections. That sections supplies, 10HP, 25HP, 40 HP and there is one 100HP motor.
The 100HP is a machine that is started randomly by the production crew. For argument sake, the 100HP has a FLA of 125A, and inrush of 6X.
The Long Time on the 1200 breaker is set at 1200A with time delay.

The fastest setting for the maintenance switch I would use would be:
(6 x 125A) + 1200 =1950A

You can't arbitrarily say 'to a reduced IE value or lower PPE rating'.
In this example short current is around 22KA, and arcing current is around 14K, but the breaker already has Instantaneous set at 10x ( or 12KA).
So under a dead fault condition, trip times would be the same and IE would not change.

The advantage of the Maintenance Switch is that it acts on the INT trip curve without time delay even on faults that produce less than full arcing current, and it reacts on the beginning end of the fault inrush (although that is probably measured in nanoseconds of difference).

So my goal is to set it as low as possible, but still allow normal plant operations without tripping feeders off the line for normal current inrush like starting motors.


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 Post subject: Re: Typical Settings for Switchgear Maintenance Switches
PostPosted: Mon Aug 05, 2019 10:39 am 
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Joined: Tue Oct 26, 2010 5:00 pm
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Location: Maple Valley, WA.
Another way to do it is to see what the maximum full load current is of the 1200 ampere feeder breaker to the MCC. Then add the 100 HP motor inrush current. Then total the current and add a safety factor.

Assume that the feeder breaker sees a typical full load current of 825 amperes.

(825 + (6 x 125)) x 1.1 = 1,733 Amperes

Min AF Maintenance Setting for the breaker is 1.5 x 1200 = 1,800 A. Therefore, set the breaker maintenance pick up to 1,800 A (1.5X)

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 Post subject: Re: Typical Settings for Switchgear Maintenance Switches
PostPosted: Mon Aug 12, 2019 8:35 am 

Joined: Mon Dec 07, 2015 9:45 am
Posts: 33
Location: Massachusetts
JBD has the right answer. You are only required to set the INST to below the available arcing fault current with no delay.

If you are able to do this, then there is no reason to deviate your LT/ST settings from what they are normally set at.
If this is not possible, then you would want to set your STPU and STD as low as possible at that arcing fault current.

You could then lower the ST settings to tightly coordinate against your feeder protective device curves, and similarly set your LT setting to what your normal operating load is (ie you may have it set to 1200A but your max load may never actually go above 900A, so set it to 900A). However this is up to you and the facility to decide, as it's not required and the argument could be made that it is not a necessary change to protect from arc flash concerns.


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 Post subject: Re: Typical Settings for Switchgear Maintenance Switches
PostPosted: Mon Aug 12, 2019 10:16 am 
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Joined: Mon Sep 26, 2011 2:17 pm
Posts: 20
Location: Waggaman - Cornerstone Chemical Company
The largest motor inrush should be the determining factor, so your logic is valid. Not a whole lot of difference between 1950 and 1800A as one of the other users offered, either would be fine.

One other thing - we have a procedure for using these switches, so we added a "notification" line for the production group to hold off any motor start activities on the MCC being worked on.


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 Post subject: Re: Typical Settings for Switchgear Maintenance Switches
PostPosted: Tue Oct 29, 2019 6:49 am 

Joined: Tue Oct 29, 2019 6:23 am
Posts: 19
I would suggest various good answers were posted.
You should understand two things:
1) what is the lowest expected arcing current?
2) what is the highest operational transient current that may be expected when the ERMS is enabled?
Your setting needs to be in between those two values accounting for all possible tolerances. And the lower the setting/threshold is, I.e. the closer to 2) above, the more robust (reliable) the protection is. Generally, instantaneous protection is not shown in TCCs as inverse, so it is assumed a lower threshold does not cause faster protection at higher currents. However, that may not be entirely true, in some cases IOC protection may have a slightly inverse operating characteristic, it is just not shown in the TCC and the difference/slope may be small.


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 Post subject: Re: Typical Settings for Switchgear Maintenance Switches
PostPosted: Tue Oct 29, 2019 3:12 pm 

Joined: Tue May 29, 2018 8:19 am
Posts: 29
Strictly speaking, JBD is correct. A couple of other considerations...

1. At least one manufacturer advertises their maintenance mode switch is faster than the normal instantaneous. If so, setting the maintenance mode pickup below the instantaneous would provide some advantage in reducing IE.
2. A note of caution: At least one manufacturer's maintenance mode switch replaces the normal trip curve with maintenance mode curve - instantaneous only - when active. So it is possible to make the situation worse by setting the maintenance mode pickup incorrectly.

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