Showing posts with label PICS. Show all posts
Showing posts with label PICS. Show all posts

ARC FLASH Hazard - Flashover

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We know that when two conductors are separated and a strong enough field is present between them, arcing occurs. An arc flash is the uncontrolled release of energy when a phase conductor comes into contact with ground or two phase conductors get shorted.
It becomes a hazard when a large enough energy is released in a short enough duration of time. 

➤ Arc flash is most likely to occur during maintenance activities, when a person might accidentally short two points at different potentials.

IEEE 1584 – Guide for performing arc flash hazard calculations and calculation of incident energy.

  • Category 0: 0 - 1.2 calories/cm2
  • Category 1: 1.2 - 4 calories/cm2
  • Category 2: 4 - 8 calories/cm2
  • Category 3: 8 – 25 calories/cm2
  • Category 4: 25 - 40 calories/cm2
➠ The first line of defence to any hazard is by engineering solutions to minimize the likelihood or consequence of the event occurring.
➠ Arc flash incident energy at a point is mainly dependent on three things – fault current, fault duration and distance of that point from the point of fault.
➠ To minimize fault current, we can have fuses. Protection settings can be set to low to minimize the tripping time thus limiting the fault duration (although this may result in false trips). 
➠ We can reduce the incident energy (say at a point outside a MCC) by providing ‘other means’ for the arc flash energy to get dissipated. So, if we provide a vent on top of a MCC, the arc flash energy has more space to dissipate energy than just going straight out. This would reduce the arc flash boundary (defined as the point at which there is a 50% chance for an unprotected person to get 2nd degree burns).

The object of this exercise is to raise your awareness of arc flash hazard.

  • If you leave equipment alone it is unlikely to arc, and due to probability, if it does, it is most likely that anybody is there.
  • You are right, arc flash is more likely to happen when individuals are there, because of the very activities they are performing, be it because they cause a short directly, or because there is a faulty component which is disturbed by their activities.
  • First line of defence is not to work on or near live equipment
  • Second line of defence is in the design as you can never design out the hazard, only reduce it.
  • Last line of defence is PPE.
  • I was at an ABB Seminar the last couple of days and I came across this:
  • Ultra-Fast Earthing Switch (UFES) – It is a sacrificial device to protect equipment in the event of an arc fault. What this essentially does is convert the arc fault into a “bolted” fault, by connecting the phase to earth in 2ms, so that fault current flows through this device instead of the actual fault location. It has a micro-gas generator (which are also used to inflate air bags in vehicles), which ruptures, resulting in the moving contact travelling like a bullet to bond with the fixed contact and created a bolted fault.

➤The device costs around $10k and the replacement cost is somewhere around $4K. Probably a lot cheaper than having to replace the whole switchgear after an arc fault occurs.

All Phones Sim Card Sizes



Mini SIM: This SIM card has been around for many years now and can be found on older smartphones or certain feature phones even today. The size of the SIM card is pretty big and if phones like the iPhone 3GS and Nexus 4 are some examples of the phones that relied on the mini SIM card.
Micro SIM: The micro SIM is pretty much the standard these days for the majority of smartphones. Pretty much all Android smartphones at the moment accept the micro SIM as the SIM card of choice. It is noticeably smaller than the mini SIM and has helped to contribute to the creation of thinner smartphones. Phones such as the iPhone 4, iPhone 4S, Nexus 5, and Samsung Galaxy S5 are examples of phones that use the micro SIM.
Nano SIM: The nano SIM is currently the smallest SIM card available at the moment. At one point in time, there was a bit of contention between Apple and Nokia regarding the nano SIM in which Apple was looking to make the nano SIM the new standard in SIM cards.
The nano SIM is much smaller than the micro SIM and it pretty much leaves just the chip of the SIM card exposed, where the micro and full-sized SIM cards still had a bit of plastic surrounding the edge.
Apple has since adopted the nano SIM as the standard on its mobile devices starting with the iPhone 5, and Nokia has since followed suit despite their initial reluctance with phones like the Nokia Lumia 1520 and Nokia Lumia Icon, just to name a few. Android OEMs such as HTC have also started to use the nano SIM with the HTC One M8.
Embedded SIM: This is another type of SIM although its use is aimed more at M2M applications rather than consumer products like smartphones or tablets.
At one point in time it was rumored that Apple had considered the idea of incorporating embedded SIMs into its products in which customers could simply choose their carrier at an Apple store and have it activated there and then.
Usage of embedded SIMs can be found in vehicles where carriers such as Vodafone have supplied embedded SIMs to the upcoming 2015 Audi and Volkswagen models.

Design An EEG at home


This is my design for a DIY saline based electrode set. I chose the saline approach because it is more convenient than using conventional electrodes. Conventional electrodes require a conductive paste which can be expensive, time consuming and messy.
Commercial electrodes are made from precious metals because they conduct well and do not react (ie. rust). I'm hoping the saline approach will counter any impedance gained from using inexpensive metals.
This is an ongoing experiment. Once I have some comparative data I will be able to tell you how well they work. They are sensitive enough to measure ocular (eye) movements which is a good start.
There is one design flaw however. The small screws in the coax plugs rust.


COMPONENTS:

        ITEM                                             Cat no.       AMOUNT      
  • Head band                                                                     1                  
  • Elastic                                                                         30cm           
  • 5 core shielded cable                              W2040              2m               
  • Twin core shielded audio cable                 W2034              1m               
  • Male plastic coax plug                            P2021                5                  
  • Sponge ear plugs                                                         packet of > 5      
  • Heat shrink tubing 4.8mm                       W4104             1.2m            
  • Heat shrink tubing 6.4mm                       W4104             1.2m            
  • Table salt                                                                       1                  
  • 5 pin DIN plug and socket *                                             1                           
  • Insulation tape *                                                             1                                        
HOW TO MAKE ELECTRODES AT HOME:
  • The electrodes are made from coax connectors and ear plugs (sponge).
                                                               

  • The pin assembly inside the plug is removable and is clamped onto the end of the cable with a screw.


  • The pin is inserted into a hole in the back of the sponge. You can use something like a bamboo skewer to prepare the hole.
  • The plug is then reassembled through the head band.
  • The sponge is pre-soaked for at least a day in a saline solution.
  • I also use one of these for my DRL (driven right leg).
CABLING:

Initially I considered using audio jacks to connect my cables to the eeg but decided against it because they are easy to snap (especially when somebody walks away while still wearing the electrodes).
Instead I opted to use a 5 pin din connector.
I split the cable progressively down from 5 pair to a single core for each electrode. I was careful to continue the shielding and used heat shrink and tape to insulate the soldered joins.


HEAD BAND:
  • The head band is a wide black plastic ladies hair band purchased from a chemist.
  • I did not own a drill bit large enough to fit the coax plugs so I used the biggest bit on hand and filed the rest out with a circular rasp.

  • It is important to remember that the head band rests a good 3cm (1/2 inch) away from the scalp when the electrodes are inserted. Take this into consideration when choosing where to drill your holes.
  • I used the standard international 10/20 system positions.
  • The driven right leg is placed in the middle of the scalp (CZ).



SALINE SOLUTION:

The saline is prepared by mixing a whole heap of table salt and water (I don't know how much is optimum, but more is better than less). It is best to use distilled water if you can.

For Any Instruction you can ask me any time by commenting below......


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