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Modelling HRC fuses for arc flash analysis
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Author:  mnewman [ Thu Mar 26, 2015 1:29 am ]
Post subject:  Modelling HRC fuses for arc flash analysis
IEEE1584-2002 Section 4.6 Step 5 states that if only fuse average melt time is published, add 15% up to 0.03 sec and 10% above 0.03 sec to determine total clearing time.

Where:
- Average melt time is equal to pre-arcing time.
- Total clearing time is equal to pre-arcing time plus arcing time.

Some LV fuse manufacturers publish a +/- 10% current tolerance for their pre-arcing time curve.
The above IEEE1584-2002 Section 4.6 Step 5 does not take this current tolerance into account.
The effect of i2t due to d.c. and a.c. current decrements is also not mentioned in IEEE 1584-2002.

What is the established practice when modelling HRC fuses for arc flash analysis to IEEE 1584-2002 ?
Author:  john.a [ Sun Mar 29, 2015 11:19 pm ]
Post subject:  Re: Modelling HRC fuses for arc flash analysis
You correctly note that IEEE1584-2002 Section 4.6 Step 5 states that if only fuse average melt time is published, add 15% up to 0.03 sec and 10% above 0.03 sec to determine total clearing time.

What I do not understand is this, " Should the +15% and +10% quantities be percentages of time or current?" If they are percentages of time then there is a peculiar phenomenon either side of 0.03 sec.
At 0.299 sec melting time the total clearing time is 0.299 * 1.15 = 0.344 sec;
At 0.301 sec melting time the total clearing time is 0.299 * 1.15 = 0.331 sec, slower than the above total clearing time.
If the +15% and +10% quantities are percentages of current then this phenomenon is eliminated.

Can any readers clarify this quandary?
Author:  Jim Phillips (brainfiller) [ Mon Mar 30, 2015 3:55 pm ]
Post subject:  Re: Modelling HRC fuses for arc flash analysis
Good catch. This is actually something I have been discussing with a few people. What some software companies are doing is using a time of 0.01 seconds for CL fuses in the current limiting region since the time current curve "buries" itself into the horizontal axis and it's difficult to know what the actual time would be.

The discussion was similar to your question, if 0.01 is used, does a margin need added on top of it. I think right now the answer is no in this region. Mostly because a CL fuse is required to begin clearing the fault before the first 1/4 cycle and it must clear the fault before the first half cycle both of which are below 0.01 seconds.

However, in the non-current limiting region you use the TCC and you are correct, depending on whether it is by time or by current, you can get completely different results. It would be best to consult the fuse manufacturer. I can't imagine any fuse manufacturer that would not be able to give better guidance. I'm sure this will be sorted out in the next edition of IEEE 1584.
Author:  arcad [ Wed Apr 01, 2015 8:39 am ]
Post subject:  Re: Modelling HRC fuses for arc flash analysis
john.a wrote:
You correctly note that IEEE1584-2002 Section 4.6 Step 5 states that if only fuse average melt time is published, add 15% up to 0.03 sec and 10% above 0.03 sec to determine total clearing time.

What I do not understand is this, " Should the +15% and +10% quantities be percentages of time or current?"


The +15% and +10% quantities are indeed the percentages of time. The IEEE 1584 4.6 Step 5 reads "For fuses, the manufacturer's time-current curves may include both melting and clearing time. If so, use the clearing time. If they show only the average melt time, add to that time 15%, up to 0.03 seconds, and 10% above 0.03 seconds to determine total clearing time. If the arcing fault current is above the total clearing time at the bottom of the curve (0.01 seconds), use 0.01 seconds for the time". It is the established practice when modelling HRC fuses for arc flash analysis to IEEE 1584-2002.

Also, the opening time tolerances differ between fuse designs and the percentage of the overload relative to the amp rating. Incrementing the time according to IEEE 1584 guidelines should effectively compensate for the margin of error in published time-current curves in most cases.

Arc Flash Analytic (AFA V5.0) software program automatically applies the required 15% and 10% time increments per IEEE 1584 when calculating incident energy and arc flash boundary. The program also provides for adding fuses and circuit breaker data including time-current characteristics for the devices that are not already featured in the built-in protection device library. This feature allows factoring in the actual time-current curve margin of error and/or temperature and altitude derating factors on discretion of the designer.
Author:  mnewman [ Fri Apr 10, 2015 12:14 am ]
Post subject:  Re: Modelling HRC fuses for arc flash analysis
1. Fuse manufacturers have informed that a +/- 10% current tolerance applies to their fuse average melt curve ( per-arc curve ).

2. IEEE1584-2002 Section 4.6 Step 5 states that if only fuse average melt time is published, add 15% up to 0.03 sec and 10% above 0.03 sec to determine total clearing time.

Is the following method correct for modelling fuses used in arc-flash analysis to IEEE 1584-2002 ?

Step 1. Add 10% current to the time-current co-ordinates of the fuse average melt curve.

Step 2. On the curve produced by Step 1. add 15% time up to 0.03 sec, and add 10% time for times above 0.03 sec
Author:  mnewman [ Thu Apr 16, 2015 7:30 pm ]
Post subject:  Re: Modelling HRC fuses for arc flash analysis
The 2011 Amendment 2 of IEEE 1584, Clause 4.6 Step 5 states :
“If the (fuse) curve has only the average melt time, add 10% plus 0.004 seconds to that time to determine total clearing time. If the total clearing time at the arcing fault current is less than 10 milliseconds, use 0.01 seconds for the time.”

Should the +10% quantity that must be added to the average melt curve be a percentage of time or a percentage of current?
I ask this question because European and North American fuse manufacturers have informed that a +/- 10% current tolerance applies to their published pre-arcing ( melt curves ).
Author:  Jim Phillips (brainfiller) [ Mon Apr 27, 2015 12:20 pm ]
Post subject:  Re: Modelling HRC fuses for arc flash analysis
I have seen a few different interpretation so I checked with a colleague at Mersen - (fuse manufacturer) and here is the response:

The curves are +/- current, usually 10% but there are some variations in the percentage especially at the current extremes (curve time over 2 minutes or close to 0.01 seconds).

Hope this helps.
Author:  john.a [ Tue Jun 02, 2015 12:23 am ]
Post subject:  Re: Modelling HRC fuses for arc flash analysis
Can readers please advise how they apply Clause 4.6 Step 5 of IEEE1584 2nd Amendment of 2011 that specifies, "add +10% plus 0.004 seconds to that time" ( That time being the average melt time)?

Do you add the 10% to the average melt curve in the direction of current or in the direction of time?

Do any readers add 10% in the direction of current to create the maximum melt time curve then apply the following approximation over the entire current range:
total clearing time = maximum melt time, neglecting the 0.004 seconds that the 2nd Amendment calls for?

I can confirm Jim Phillip's advice that Mersen, along with Littelfuse, Bussmann, Siba, GE and Comeca advise a tolerance of +/- 10% of the average melt time curve 'in the direction of current'.

The attached curve exhibits the differences between the fuse characteristics obtained as follows:
1. +10% in the direction of current /_ 10% in the direction of current (dark green curve),
2. +10% in the direction of time plus 0.004 seconds/- 10% in the direction of current (red curve) &
3. average melt time curve (lime green curve).

Attachments:
File comment: The characteristic obtained by applying Clause 4.6 Step 5 of IEEE1584 2nd Amendment of 2011 gives the blue curve. This curve is completely obscured by the red curve which has the IEEE amended curve as an upper limit and a lower limit at -10% of the average melt curve.
250A GEC HRC FUSE COMPARE.pdf [9.71 KiB]
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