Explosions in Electrical Vaults
W. Z. Black
School of Mechanical Engineering
Georgia Institute of Technology
ICC
March 2008
What’s the Problem?
Explosions in manholes may be infrequent
but are not uncommon
- One utility has 57,000 manholes with 50 events per
year (1:1000)
- Another utility has over 250,000 manholes with about
1000 events (many were minor) per year (1:250)
Damage to equipment can be significant and
potential injuries can be severe. Several deaths
and numerous injuries have been reported
If explosions become public knowledge, media,
PSC and political entities will become involved.
Litigation is a possibility
Source of Explosions
Short Circuit Events – energy release
before relay cuts off current can be huge.
Ignition of Combustible Gases – gas
explosions release a large amount of
energy.
Combination of Arcing Event and Ignition of
Gases – arcing event can ignite
combustible gases which adds to explosive
energy.
Strength of Explosions
Gas Explosion - for typical size manhole
filled with stoichiometric CH4, explosion
lasting for 0.5 sec
– Energy released by explosion is approx 40
MJ or average power is 80 MW
Arcing Fault – assuming a fault on a 7,200V
cable with an available fault current of 5,000
amps for 10 cycles
– Energy released by arcing fault is approx 6
MJ or average power is 36 MW
(one stick of dynamite has energy of 2 MJ)
Complicating Issues
Vault is rigid and unable to absorb energy. All
generated energy must go into the air in the
vault.
Event is very rapid and any mitigation design
must react rapidly. Timing is very critical.
– Relaying system reacts within 0.1-0.2 sec
– Flame speeds limit explosion to 0.5-1 sec
Vault is basically closed and venting is limited.
Attempting to bolt down cover is poor decision.
Explosive forces can break most bolts & welds.
Manhole Events - Past History
Several deaths have been reported.
Other events have resulted in personal
injury to passers-by.
One utility reports more than one event per
day.
Several events have resulted in litigation –
potential financial loss can be substantial.
Manhole cover is the weakest link. Cover
can be propelled over great distances and
can become a lethal projectile.
Concern is Public Safety &
Damage
Electrical Faults
Gas Explosion Tests at EPRISolutions – Lenox MA
Energy of Gas Explosion
EPRI Lenox Gas Explosion Tests
Bad Publicity is an Unwanted
Reality
Value of Software
Eliminates need for expensive tests
Can predict trends and it permits the
evaluation of potential of safety devices
such as:
–Restraints/tethers: what forces must they
withstand?
–Vents: what size, shape, weight would be most
effective?
–Gas reduction schemes: will gas-displacing or
gas-venting schemes be effective?
Software Capabilities
Geometry of vault and vent is flexible
For arcing faults, voltage and current
waveforms are arbitrary
For gas explosion, gas and concentration are
arbitrary
Program calculates pressure, temperature
inside vault; position of cover and forces on
any restraining device as a function of time
Present focus is on design of mitigation
schemes
Software Framework
Program consists of a number of interlaced
sciences:
Heat Transfer: energy liberated from the explosion or the
arc and transfer to the air in the vault
Fluid Mechanics: Flow of gases from the vault and past the
manhole cover
Combustion Chemistry: Reaction of the combustible gas
mixture
Thermodynamics: Complex relation between the properties
of the gases in the vault to the heat liberated by the
explosion
Mechanics: Calculation of the motion of the cover when it
experiences the flow of gases and the influence of the tether
Schematic of Tethered Cover
Recent Experimental Tests
Major objective was to design a cover
restraint system for two new transmission
circuits
Tests consisted of a single above ground
test to determine magnitude of arcing fault
Above ground test was followed by two
tests in an underground vault
cable fault
splice fault
Initial Above-Ground Test
138 kV Cable
2250 kcmil copper conductor
850 mil XLPE insulation
Corrugated copper shield
PE jacket
Faulted with a bolt driven between
conductor and shield
Cable Before Test
Cable After Test
Vault Test – Splice Fault
Peak voltage
2550 V (RMS)
Peak current
107.3 kA
Arc duration
308.4 ms (18.5
cycles)
Amount of gas burned is unknown, but
some smoke exhausted from chimney
Measured pressure rise 7 psi
Calculated pressure rise 11 psi (no gas
contribution)
Splice Before Fault
Splice After Fault
Vault Test – Cable Fault
Peak voltage
1400 V (RMS)
Peak current
172.4 kA
Arc duration
313.2 ms (18.8 cycles)
Amount of gas burned is unknown, but
some smoke exhausted from chimney
Measured pressure rise 8-9 psi
Calculated pressure rise 9 psi (no gas
contribution)
Cable Before Fault
Cable After Fault
Conclusions
Due to available energy released during
manhole events, potential for damage to
equipment & danger to personnel can be
significant
A manhole event software package can help
design reasonable mitigation schemes that
will address the case of gas explosions
consisting of a number of gases.
The software has shown that it can
successfully predict the worst case conditions
during a demanding combination of gas
explosions.