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| reducing dangerous condition https://brainfiller.com/arcflashforum/viewtopic.php?f=12&t=2505 |
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| Author: | bure961 [ Wed Oct 24, 2012 6:08 pm ] |
| Post subject: | reducing dangerous condition |
I do not completley understand what causes the IE in hrc to be 2-3- or a hrc4. My question is if I am doing troubleshooting in a motor starter cabinet or mcc and the cabinet has a hrc 4 label on it can i open the cb and disconnect the load and work in the panel ? Will disconnecting the load before beginning work lower the IE ? |
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| Author: | PaulEngr [ Fri Oct 26, 2012 5:55 pm ] |
| Post subject: | |
bure961 wrote: I do not completley understand what causes the IE in hrc to be 2-3- or a hrc4. My question is if I am doing troubleshooting in a motor starter cabinet or mcc and the cabinet has a hrc 4 label on it can i open the cb and disconnect the load and work in the panel ? Will disconnecting the load before beginning work lower the IE ? Actually yes disconnecting the load CAN lower the incident energy if it's an inductive or capacitive load such as a motor. When there is a fault, energy flows simultaneously from BOTH the line and from energy storage devices such as motors. So if you eliminate the motor the incident energy goes down. HOWEVER, the general approach is to look at a worst case scenario. The easiest way to understand the CONCEPT is that if you have an arcing fault, the power flowing into the air gap where the arc is happening, proporitional APPROXIMATELY to volts times amps (it's not exact), is emitted as heat. The limit on the amount of current is the short circuit current of the system which is often thousands of amps. Cable and transformer impedances are generally what reduces this. The amount of damage caused by the heat depends on how long the arc occurs. And power multiplied by time is energy. Now the last factor here is that the energy is generally radiated in all directions so the farther you are away, the lower the amount of heat over an area of your skin. To give you a perspective, holding your hand in the flame over a disposable lighter for 1 second, 1 inch away is about 1 cal/cm^2. If you want to reduce incident energy, either move farther away, speed up the circuit breakers or fuses (reduce time), or use smaller transformers (lower available fault current). The old trend of setting the protective systems as high as practical is dead. The current strategy is to set them as tightly as possible. Also, multiple smaller transformers and circuit breakers are a better strategy than putting in one large one. And installing equipment that is not maintainable (e.g. sealed breakers) is an effective strategy. |
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| Author: | Ron Baggett [ Mon Oct 29, 2012 9:35 am ] |
| Post subject: | |
bure961 wrote: I do not completley understand what causes the IE in hrc to be 2-3- or a hrc4. My question is if I am doing troubleshooting in a motor starter cabinet or mcc and the cabinet has a hrc 4 label on it can i open the cb and disconnect the load and work in the panel ? Will disconnecting the load before beginning work lower the IE ? PaulEngr gives a very good reply. If you are opening the main disconnect for the MCC line-up, you have disconnected all the loads. Caution! Some MCC's have more than one main disconnect and some MCC's have no main disconnect (MLO). There a slight possibility of an incident occuring if a foreign power source is introduced to the disconnected loads. The more likely source for an incident is upstream of the main breaker that might expand beyond that section into the adjoining sections. Depending on the severity of the incident and your distance from the incident, you may or may not be injured. The buckets themselves will provide some protection if the distance from the incident is substantial, or the incident duration is very low. If the incident energy is high and nearby, the buckets become shrapnel. The best thing would to disengage the line side disconnect for the transformer feeding the MCC. Your company policy will determine the level of liability they are willing to accept. It is common for companies to allow work on a downstream bucket if the Main disconnect for the line-up has been opened because with your level 2 PPE, you are reasonably safe working within the buckets. Keep in mind you could still be struck by a toilet seat falling from the MRI space station. |
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| Author: | jghrist [ Mon Nov 05, 2012 11:49 am ] |
| Post subject: | |
Paul and Ron's answers are correct, but I think the simple answer is: Disconnecting the load will not significantly reduce the IE. You cannot reduce the HRC by doing so. The energy mostly comes from the MCC source, not from the load. |
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