[Milsurplus] Mil generator

[Richard Brunner]

OK, a bit more on grounded vs ungrounded systems...

Grounded systems:
* With circuit breakers or fuses in the feeders, a short or single 
ground fault will take out that circuit immediately.  There is no 
reprieve - you have to fix it.
* In event of a high to low voltage winding failure in the feeder 
transformer, which is very rare, the grounded system provides some 
protection from the high voltage appearing on the lv system.
* In event of a ground fault, the short sees full available 
short-circuit current, which may be 5,000-20,000 amps, creating a bad 
flash and burn.

Ungrounded system:
* With the ungrounded system, in event of a single system ground, you 
can continue to operate until you get a second ground fault, at which 
time the circuit breakers or fuses operate.  This gives you time to find 
the first ground fault.
* System transient voltages can be higher, but that isn't a problem with 
modern insulation.  Modern insulation typically has dielectric strength 
of 200-300 volts per mil, so you don't need much lv insulation.  LV 
insulation is usually much thicker for abrasion resistance and 
manufacturing tolerance.  For example, a good quality new lv cable might 
withstand 50 kv for a minute.
* In event of a ground fault, the 1'st ground fault current is limited 
to the system capacitance to ground, and is at most an amp or two.  The 
2'nd ground fault is big.  This is safer for the electrician who drops a 
screwdriver, etc, because he isn't blinded, burned, and deafened by the 
big short-circuit blast.

Philosophy:
The NEC has been pushing grounded systems for a long time, citing 
greater safety, etc.  I think this is a bit benighted, but it won't 
change any time soon.  They make allowance for applications where 
continuity of service is very important, such as in chemical 
manufacturing where loss of power is a catastrophe.

Someone will inevitably say, "With ungrounded systems there is danger of 
Ferroresonance." (When something happens that he can't explain, it's 
always Ferroresonance)  Ferroresonance is when a transformer or inductor 
is grossly driven in and out of saturation, acting as a switch as the 
inductance appears and disappears, trapping charges on the line, causing 
line voltage to go 1, 3, 5, 7 per unit until something flashes over. 
Ferroresonance is exceedingly rare - in 40 years I never saw it.

So, don't worry about an ungrounded system on your milsurplus generator...

Richard Brunner, AA1P