The library closed at 8 PM last night so I was not able to finish my post.
There is an out of print
book titled Collier Electrician published by the British National Coal Board on how protection of high energy electrical cords works. What British National Coal Board and their American ( Mine
Safety and Health Administration ) and
German couterparts found was that what was causing a lot of fires and essentially all electrocutions in mines was that as a
cord was flexed one or more
wire strands would break and start working their way through the
insulation. If the
strand worked its way through to the other side a
short circuit or smoldering arcing fault would occur. If the
wire strand worked its way to the surface and electrocution would occur, usually when somebody stepped into
current flow into moisture along the floor.
What
conductor shielding and automatic continuous monitoring of the equipment
ground does is to assure that a stray
wire strand turns into a
ground fault and electricity leaks from the
cord. What resistance grounding does is to keep
ground faults from burning themselves
clear or turning into a smoldering arcing fault. Resistance grounding also gives protective
ground fault relays time to respond to the fault and
disconnect power and also when the
power is off the electricians can find and remove the fault.
Ground fault protection at both the branch
circuit and
feeder level is supposed to trip on branch
circuit faults so that 2 relays and 2
circuit breakers are disconnecting the fault - this guarantees redundancy in the protection
system.
There is a scenario where there can be a catastrophic bolted fault such as when a machine accidentally runs over a
power cord ( about once every 10 years in the U.K. ) . This approximately corresponds to how you can
choke to death on your breakfast food.
Ground fault relays need time to
disconnect ground fault both because of the time that it takes to sense the fault and for a
circuit breaker to trip and because there has to be an INTENTIONAL delay to allow motors to start and stop. Resistance grounding provides this time by absorbing potentional
arc flash energy and by limint the fault to a relatively low
level of
current compared to a bolted or arcing fault.
When a motor starts there is an initial phantom "
ground fault" because of charging of the winding
insulation capacitance to
ground. When the starting
switch of a single
phase motor opens and when all motors are turned off ( both single and three
phase ) winding
inductance forces a phantom "
ground fault" to flow through winding
insulation capacitance to
ground what is known as a second order transient
ground fault.
For a 120 volt
ground fault
circuit interrupter
receptacle or
circuit breaker the intentional time time delay needs to be a minimum of 1/10th of a second.
Underwriters Laboratories allows a maximum inverse time delay of
Ground fault current maximum time delay
6 milliamperes 5.59 seconds
10 mA 2.69 sec
20 mA 1 sec
30 mA 0.56 sec
In a wet location such a as a bathroom this is acceptable because hair dryers normally do not get wet. In a waste water treatment room of a factory this is unacceptable and would wrap somebody's arm around their head. This is because electronic devices and so forth are going to have some amount of moisture inside that would leak some amount of 120 volts back through the equipment
ground.
What automatic continuous monitoring of the equipment
ground does is to say that it is the equipment
ground that protects people against shocks so shut of the
power when the
ground fails.
I have also been shocked by a perfectly 480 volt
hoist motor in a bridge crane in a dry location. This is because motor winding
insulation capacitance will leak some amount of 480 volts to
ground even if the
insulation is perfect. That is why U.S. National Electrical Code requires bridge cranes to grounded both through a dedicated
runway conductor and the support rails and wheels.
I am pretty sure that some of you have also been shocked by a broken
ground wire.
Mike Cole