I have a lot of objections to the decision to lower the maximum voltage for unshielded cables to 2,400 volts phase to phase. This seems to be based on BS that contradicts actual experience in tens of thousands if not hundreds of thousands of installations.
In the May 2005 issue of EC&M magazine the reasons given were safety, reliability, and inadequate training. Let's go over actual experience:
1. 120 volts is just as deadly as 2,400 volts and 4,800 volts. Just ask anybody who has died from contact with 120 volts. In fact, most electrocution deaths involve 120 volts. Same goes for getting tossed off a ladder by 277Y480 volts and breaking your neck.
2. Shielding will not stop mice from chewing on the wires. Mice and rats can chew through steel and concrete when they are hungry enough and smell food.
3.a. Tens of thousands of airports have 6.6 amp and 20 amp series lighting circuits that operate with unshielded conductors up to 4,600 volts. This has never been a reliability problem or a safety problem in several decades. You just simply do not work on a series lighting circuit while it is energized. Period.
3.b. The plugs that are used on the primary side of the isolating transformers for individual lights would have to be completely reengineered to work with shielded cables. Same for the isolation transformers for individual bulbs. These products would also have to be backwards compatible with the old unshielded stuff - I would really like to see that done.
I will even take a bet that the airports and Federal Aviation Administration will appeal this change. Using shielded cables would actually create more problems than any that shielded cable alledgedly solves.
4. One reason for using unshielded cable is that you do not need stress cones and so forth at terminations. That is, Keep It Simple Stupid.
5. All failures of 5 KV and 8 KV unshielded conductors can be attributed to the following:
Insulation nicks
Lightning
Old age
Mice chewing on the wires
Insect nests
Somebody working on it energized
Not all of the wires strands in a connection not conducting leading to an overheated connection
Physical accidents such as somebody digging up the line with a backhoe
Sabotage
Suicide
Ferroresonant overvoltage on ungrounded systems which is how 480 volts ungrounded can have 2,000 volts phase to ground - the solution is high resistance or medium resistance grounding.
Now I want somebody to tell me how cable shielding will fix any of these problems.
6. International Exposition Center in Cleveland, Ohio has 4,800 volts ungrounded ( except for the gorund detecting voltmeters ) in 5,000 volt type MC cable that does not have shielding of individual conductors. All of the paper insulated cables installed during World War 2 on this system have been disconnected because they were no longer needed. The rubber insulated cables installed after the war by General Motors have been very reliable. The only cable fault in the past 10 years was that the underground cable to the jet hanger blew out. That was attributed to old age and water infiltration.
A lot of the automotive factories in the Cleveland area also have this type of distribution.
7. The electrical utility in Buffalo, New York has 4,800Y8,320 volts solidly grounded running overhead as unshielded conductors triplexed and then strapped with bare steel wire to a steel messenger wire. Has been very reliable for several decades in spite of sun exposure, salt spray from the roads, yada yada.
8.a.?With the exception of systems that have a spark coil plugged directly onto each spark plug, every spark ignition engine has unshielded cable running at very high voltages.
8.b. General Motors High Energy Ignition operates at well over 8,000 volts.
8.c. Every auto mechanic that there is knows how to SAFELY work on these high voltage unshielded cables. You work on the d@mned stuff while the engine is not running.
9. Most auto mechanics got their high voltage training from somebody other than the U.S. military, IBEW, or Unorganized Auto Wreckers. These 3 places are not the only sources of electrical knowhow. I cannot get training from any of these 3 sources yet there is way for me to get the training.
10. Grainger sells hot line gloves, face shielding, grounding jumpers, the whole 9 yards.
11. My first pair of hot line gloves saved my life 3 months after I bought them. I was removing a rather heavy panelboard cover and 1 of the screw operated blind hooks was wrapped around a 20 amp 120 volts ciruit. The only thing that I could do was to dig a cover corner through some paint on the box and then wait 30 maybe 45 seconds for an alledgedly fast acting plug fuse to blow. That was the longest 30 seconds in my life.
12. 8 KV insulated wire has been used in overhead distribution up to 7,970Y13,800 volts as single conductors in free air because it is resistant to squirrels, tree branches, and other sources of short circuits. In order to use shielded wire with an overhead distribution transformer would require reengineering the transformer plus a lot of extra cost items such as a pothead and dead front elbow that would drive up the cost of a simple safety and reliability method.
13. In the case of 4,160 volt and 4,800 volt generators unshielded cables simplify ground fault protection and differential current protection of the generator. ( Differential protection is similar to GFCI but appied to each phase winding to as to also detect phase to phase leaks. ) That is, you can run 2 or 3 unshielded cables at these voltage through a window type current transformer instead of having to parallel the secondaries of relay class CTs - the latter condition is much more dependent on the accuracy of the CTs during a short circuit.
Unshielded conductor has been used successfully for several decades in quite a few applications - there is just no evidence other than emotion or politics to indicate that unshielded cable is any less reliable or safe at voltages over 2,400 volts.
Removed inflammatory statement concerning unions.
Charlie
[ June 07, 2005, 06:59 AM: Message edited by: charlie ]
In the May 2005 issue of EC&M magazine the reasons given were safety, reliability, and inadequate training. Let's go over actual experience:
1. 120 volts is just as deadly as 2,400 volts and 4,800 volts. Just ask anybody who has died from contact with 120 volts. In fact, most electrocution deaths involve 120 volts. Same goes for getting tossed off a ladder by 277Y480 volts and breaking your neck.
2. Shielding will not stop mice from chewing on the wires. Mice and rats can chew through steel and concrete when they are hungry enough and smell food.
3.a. Tens of thousands of airports have 6.6 amp and 20 amp series lighting circuits that operate with unshielded conductors up to 4,600 volts. This has never been a reliability problem or a safety problem in several decades. You just simply do not work on a series lighting circuit while it is energized. Period.
3.b. The plugs that are used on the primary side of the isolating transformers for individual lights would have to be completely reengineered to work with shielded cables. Same for the isolation transformers for individual bulbs. These products would also have to be backwards compatible with the old unshielded stuff - I would really like to see that done.
I will even take a bet that the airports and Federal Aviation Administration will appeal this change. Using shielded cables would actually create more problems than any that shielded cable alledgedly solves.
4. One reason for using unshielded cable is that you do not need stress cones and so forth at terminations. That is, Keep It Simple Stupid.
5. All failures of 5 KV and 8 KV unshielded conductors can be attributed to the following:
Insulation nicks
Lightning
Old age
Mice chewing on the wires
Insect nests
Somebody working on it energized
Not all of the wires strands in a connection not conducting leading to an overheated connection
Physical accidents such as somebody digging up the line with a backhoe
Sabotage
Suicide
Ferroresonant overvoltage on ungrounded systems which is how 480 volts ungrounded can have 2,000 volts phase to ground - the solution is high resistance or medium resistance grounding.
Now I want somebody to tell me how cable shielding will fix any of these problems.
6. International Exposition Center in Cleveland, Ohio has 4,800 volts ungrounded ( except for the gorund detecting voltmeters ) in 5,000 volt type MC cable that does not have shielding of individual conductors. All of the paper insulated cables installed during World War 2 on this system have been disconnected because they were no longer needed. The rubber insulated cables installed after the war by General Motors have been very reliable. The only cable fault in the past 10 years was that the underground cable to the jet hanger blew out. That was attributed to old age and water infiltration.
A lot of the automotive factories in the Cleveland area also have this type of distribution.
7. The electrical utility in Buffalo, New York has 4,800Y8,320 volts solidly grounded running overhead as unshielded conductors triplexed and then strapped with bare steel wire to a steel messenger wire. Has been very reliable for several decades in spite of sun exposure, salt spray from the roads, yada yada.
8.a.?With the exception of systems that have a spark coil plugged directly onto each spark plug, every spark ignition engine has unshielded cable running at very high voltages.
8.b. General Motors High Energy Ignition operates at well over 8,000 volts.
8.c. Every auto mechanic that there is knows how to SAFELY work on these high voltage unshielded cables. You work on the d@mned stuff while the engine is not running.
9. Most auto mechanics got their high voltage training from somebody other than the U.S. military, IBEW, or Unorganized Auto Wreckers. These 3 places are not the only sources of electrical knowhow. I cannot get training from any of these 3 sources yet there is way for me to get the training.
10. Grainger sells hot line gloves, face shielding, grounding jumpers, the whole 9 yards.
11. My first pair of hot line gloves saved my life 3 months after I bought them. I was removing a rather heavy panelboard cover and 1 of the screw operated blind hooks was wrapped around a 20 amp 120 volts ciruit. The only thing that I could do was to dig a cover corner through some paint on the box and then wait 30 maybe 45 seconds for an alledgedly fast acting plug fuse to blow. That was the longest 30 seconds in my life.
12. 8 KV insulated wire has been used in overhead distribution up to 7,970Y13,800 volts as single conductors in free air because it is resistant to squirrels, tree branches, and other sources of short circuits. In order to use shielded wire with an overhead distribution transformer would require reengineering the transformer plus a lot of extra cost items such as a pothead and dead front elbow that would drive up the cost of a simple safety and reliability method.
13. In the case of 4,160 volt and 4,800 volt generators unshielded cables simplify ground fault protection and differential current protection of the generator. ( Differential protection is similar to GFCI but appied to each phase winding to as to also detect phase to phase leaks. ) That is, you can run 2 or 3 unshielded cables at these voltage through a window type current transformer instead of having to parallel the secondaries of relay class CTs - the latter condition is much more dependent on the accuracy of the CTs during a short circuit.
Unshielded conductor has been used successfully for several decades in quite a few applications - there is just no evidence other than emotion or politics to indicate that unshielded cable is any less reliable or safe at voltages over 2,400 volts.
Removed inflammatory statement concerning unions.
Charlie
[ June 07, 2005, 06:59 AM: Message edited by: charlie ]
