MEMO # 4 FROM: Risto Jalava, Safety Officer of test event 2002
DATE: March 31, 2002
TO: Competition Director and Organiser of test event 2002
COPIES TO: Deputy Directors of test event 2002  
             

Power line contacts
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About short-circuit and arcing, tripping and automatic reconnection

Two or more power lines contacting each other directly or for instance by means of balloon flying wires result in short-circuit. It is sometimes enough that the lines come near each other without actually contacting. In that case the current may jump through the air. This is called arcing. Arcing in high voltage lines (10 kV and more) can make holes in balloon fuel tanks without any difficulty.
Arcing in low voltage lines (0,4 kV) is not likely to rupture fuel tanks. Contacting power line from ground or power line contacting ground directly results also in short-circuit.

(Video of arcing caused by short-circuit to ground in 20 kV line. Right-click, save  as and play with Windows Media Player. The 3 mighty flashes are worth seeing. And mp4-file for Mac users.)  What exactly happened?

Another video: arcing separates basket

If short-circuit takes place in high voltage lines, the current will be automatically switched off instanty, in less than half a second. This is called tripping. In low voltage lines that deliver power to households tripping takes longer to happen and it may not happen at all. High voltage power cable touching dry ground may not cause tripping, either. If no tripping happens, the line stays alive and dangerous.

In less than half a second after tripping there will be another electric surge if the automatic reconnection is on. If the lines are still in short-circuit, tripping happens again, that is, current is automatically switched off. After that there will be longer period of no current in lines, perhaps 30 seconds, perhaps 60 seconds, depending on the system and settings. Then another surge and another tripping if lines are still in short-circuit. The sequence may stop here or it may go on many times depending on how the system is set up.

If automatic reconnection is switched off, only the initial tripping after short-circuit will take place. There will be only one surge of electric current, the one that takes place when the lines short-circuit. Reconnection must be done manually and it is usually done only after finding out and eliminating the reason for short-circuit. The possibility for damage and injury is thereby reduced.

What happens when a balloon hits a power line
(typically a 3-cable 10-35 kV line)

When a balloon hits power lines there may be horrendous arcing or no short-circuit at all, or something between the two extereme alternative. It depends on many things.
- The balloon may press two or more cables together or it may touch only one cable.
- The basket may contact the power line in such way that all conductive materials keep away from the cables.
- The balloon may make the contact in such way that the flying wires or burner frame or the suspension wires that go around the basket inside the walls come close enough to cause arcing, or arcing may happen between the power cables and fuel tanks. The envelope is also conductive. Dry wicker is not.


I divide the collisions in 5 categories.

1) Balloon contacts power lines with only basket bottom or only one side of basket or with bottom and the upwind side of basket

Collision may result with tremendous arcing, rupture of fuel tanks or fuel hoses, uncontrollable fire, separation of basket from the envelope if the wires that connect basket to burner frame melt in short-circuit, and death of all occupants (caused by electrocution, burning or crash).The impact may sever the power lines. If the basket is not separated from envelope, the occupants may continue flight either with fire onboard or without.
It is also possible that the occupants suffer only minor injuries, or no injuries at all. Nothing remains entangled to power lines. In this case switching off the automatic reconnection does not help much because contact between balloon and power lines is very short (it shortens the time of electric discharge to half, however). If the power line cables are severed, it is better that they are de-energized as soon as possible, as they will be if automatic reconnection is off.

2) Balloon contacts power lines with bottom and the downwind side of basket

Collision may result with tremendous arcing, rupture of fuel tanks or fuel hoses, uncontrollable fire, separation of basket from the envelope if the suspension cables fitted round the basket melt in short-circuit, and death of all occupants (caused by electrocution, burning or crash).The power lines may melt through and separate. If the basket is not separated from envelope, it probably gets stuck in power cables, with fire onboard or without. It is also possible that the occupants suffer only minor injuries.That is more likely to happen if the automatic reconnection is off.

3) Balloon is descending and hits power lines with envelope above equator

The basket will reach ground and occupants are able to escape. The envelope and the whole balloon may burn.  The damage is likely to be much less
if automatic reconnection is off.

4) Balloon is descending and the basket squeezes itself between the individual cables possibly pressing one power cable under the basket bottom

Collision may result with tremendous arcing, rupture or explosion of fuel tanks or fuel hoses, uncontrollable fire, separation of basket from the envelope envelope if the suspension cables fitted round the basket melt in short-circuit, and death of all occupants (caused by electrocution, burning or crash).The power lines may melt through and separate.  It is also possible that the occupants suffer only minor injuries. That is more likely to happen if the automatic reconnection is off.

5) Balloon is ascending and contacts power lines between basket and envelope

Collision may result with tremendous arcing, rupture of fuel hoses and even tanks, uncontrollable fire, separation of basket from the envelope as flying wires are severed by abrasion or melting, and death of all occupants (caused by electrocution, burning or crash). Basket may remain stuck in the cables. It is also possible that the occupants suffer only minor injuries. That is more likely to happen if the automatic reconnection is off.

Video: arcing separates basket