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Time Current Curves Stop at 6 cycles
3

Time Current Curves Stop at 6 cycles

Time Current Curves Stop at 6 cycles

(OP)
Why do fuse time current curves stop at 0.01 seconds?





RE: Time Current Curves Stop at 6 cycles

0.01 sec = 0.6 cycles. Non-current limiting fuses don't clear until the first zero crossing. If current limiting, use I²t instead of TCC.

RE: Time Current Curves Stop at 6 cycles

(OP)
Thank you!

Will this effect selective coordination if the current at the fuse is greater than the current listed at 0.01 seconds?

RE: Time Current Curves Stop at 6 cycles

2

RE: Time Current Curves Stop at 6 cycles

(OP)
Ok- Cuky- Star given. But need a bit or explanation on how to go about delaing with it- specifically comparing the minimum melting energy with the total clearing energy of the downstream fuse.

Also- If I do not exceed 100,000amps for a 6,000amp fuse I can ignore anything below 0.1 seconds, correct?

RE: Time Current Curves Stop at 6 cycles

Hi Mbrook:

Quote (But need a bit or explanation on how to go about delaing with it- specifically comparing the minimum melting energy with the total clearing energy of the downstream fuse.)

The Selectivity Ratio Guide (Lineside to Load-side), usually provided by the manufacturer, could assist to initially select the downstream fuse size. The ratios are valid for all overcurrents and opening times, even for fuse opening times less than 0.01 seconds (0.06 cycles). If additional assurance is needed to mee the coordination between the fuses, a plot time-current curves or a short-circuit current analysis it is suggested.

Quote (If I do not exceed 100,000amps for a 6,000amp fuse I can ignore anything below 0.1 seconds, correct?)

The curve indicates that 0.1 sec (6 cycles) the fault current that trip the fuse should be ~70kA. For larger fault current in the range 70kA < Ifault < 100 kA the range of clearing time (arcing+ melting time) is: 0.1sec <tc<0.01sec. Not sure what you mean by “ignoring < 0.1 sec”.
See figure below for illustration and details

RE: Time Current Curves Stop at 6 cycles

(OP)
I'm confused though-

Quote:

"The ratios are valid for all overcurrents and opening times, even for fuse opening times less than 0.01 seconds (0.06 cycles)."


My understanding is that beyond a certain fault current the I2R energy needed to melt the down stream fuse could also impact the upstream fuse?

RE: Time Current Curves Stop at 6 cycles

During a SC two phenomena will arise in the fuse and other component connected to the circuit: 1) Thermal heat(Irms)2*R responsible to fuse or melt the metallic link of the fuse and 2) The mechanical forces driven by Peak let-through current.

When a fuse attempts to interrupt high fault currents, the fuse must be designed to withstand the tremendous pressure produced inside the fuse body as a result of the rapid vaporization and arcing of a portion of the fuse element. If a fuse tries to interrupt a fault current greater than its interrupting rating, the fuse can violently rupture. OSHA 1910.303(b)(4) & 1910.303(b)(5) and the NEC 110.9 & 110.10 contain similar language to use overcurrent protective devices and equipment with adequate IR and SCCR.

If the line and upstream fuses are selected properly within the SC operating range and Selectivity Ratio Guide as recommended and tested by the manufacturer, there should not be an issue. It is recommended the minimum rating ration of the line and load fuses should be as recommended by the fuse manufacturer in the range of 2:1 to 6:1 ratio depending upon the selected fuse type (see manufacturer table). If there is uncertain to achieve protective device coordination, it is advisable to consult with the fuse manufacturer for a specific case.

RE: Time Current Curves Stop at 6 cycles

(OP)
Thank you

I'm still uncertain though why they don't go below 0.6 cycles on the time current curve.

RE: Time Current Curves Stop at 6 cycles

0.01 seconds has been the traditional "bottom" of all time-current curves for decades. It simply depends on what the manufacturer decides to provide. Many manufacturer's fuse curves do start at 0.001 sec. If you are concerned about coordination between fuses, as mentioned before, the manufacturers can give you a ratio between fuse sizes that they have shown BY TEST will coordinate over the full fault current range of the fuses. This ratio is generally 2:1 or sometimes 3:1 depending on fuse type. As noted, a fuse in its current-limiting range of available fault current, will clear the fault within 0.5 cycles.

RE: Time Current Curves Stop at 6 cycles

There is not much appetite in the fuse industry to show the device performance under 1 ms.

Perhaps the reason for that is that the standard does not require the fuse manufacturers to publish TCC curve data below 1 ms. Other possible reasons are that CL fuse is much faster than circuit breaker, recloser, and other protective devices.

It should be noticed that the total fault current required to melt the fuse <1ms increase exponentially that covers most of the prospective short circuit available as shown in the enclosed Link and the excerpt below.

....

RE: Time Current Curves Stop at 6 cycles

(OP)
Of note, several electricians have told me to not worry about faults going to 200,000amps or as high as the available short circuit current at the transformer secondary because most faults are "arcing" or sputtering faults. Meaning faults have high peak current but low RMS current effecting the melting time of the fuse element.


Bolted faults are rare. Even if someone drills into conduit or something metal falls into a busbar the magnetic forces will push apart the two "electrodes" at the fault point. Current must then rely on the ionized air which 1) during conduction presents resistance 2) begins conducting 2/3 up the sine wave and stops conducting 8/9 down the wave. In some cases there can also be "quarter cycling"


As such anticipating bolted fault currents is unrealistic.

What do others think?


I want to have 100% discrimination for the entire system.




RE: Time Current Curves Stop at 6 cycles

Mbrooke,
Depends on voltage and equipment, but one easy (and depressingly common) way to get bolted faults is through operator error by leaving earths on when re-energising after outages. Three phase cable faults can also be pretty close to bolted.
John.

RE: Time Current Curves Stop at 6 cycles

(OP)
Good point and a possibility to consider.

RE: Time Current Curves Stop at 6 cycles

Quote (........ Basically its the total clearing time that can cause an upstream fuse to start melting?)


For the upstream fuse to melt, the minimum melting energy of the upstream fuse must be less than the total clearing energy of the downstream fuse .
See the example in the enclosed Link

Remember, for less than 1 ms (0.001 s), the coordination between the upstream and downstream fuses must be established via the fuse selectivity test ratio tables published by the fuse manufacturer.

RE: Time Current Curves Stop at 6 cycles

The fuses are RK-1 type, quite common in the US and made by every fuse manufacturer. The manufacturers have test data to validate their claim that using a 2:1 (or 3:1) ratio will ensure selective coordination and I have never heard anyone dispute that.

Bolted faults are not common, but certainly possible. You have to design for them.

If you absolutely must have full selectivity, regardless of fault current, then fuses are probably the most reliable solution - if you can accept the downsides, especially single-phasing. In general, MCCBs will not coordinate with each other due to the instantaneous trip. Some MCCBs are tested for "series coordination" with other MCCBs but there is a maximum fault current limit to this capability.

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