Technical Guide
Understanding and Using the “Main device fail to operate, use upstream devices” Option
in PTW V7.0.
Figure 1
Proper maintenance and testing of protective devices are essential to the integrity and safety of
the equipment and overall power system. When the main protective device fails to operate during
a fault, it will lead to an unintentional time delay that will affect the results of the Arc Flash
Analysis. Since the clearing time of protective devices is one of the key components of the Arc Flash
study, it is important to anticipate as to when the fault will clear for every possible scenario such as
the failure of the main protective device.
In PTW, the option “Main device fail to operate, use upstream devices” in the Arc Flash Study
Module, enables the user to see the resulting Arc Flash incident energies when a failure of the main
device occurs. The main device is defined in the program as the protective device that carries the
most fault current to the bus. Usually, this is the device that protects the equipment from the line
fault current (eg. Main breaker of the Motor Control Center or Main switchgear, etc). When this
option is applied, the program will ignore the main protective device and look for the next
upstream device to trip.
Technical Guide
At all locations where there is a main overcurrent protection device, the incident energy will be
calculated assuming the following:
1.
2.
3.
The arc flash event occurred at the bus or at the line-side of a protective device.
The main overcurrent protection device (MOCPD) failed to operate, therefore it is ignored
The first upstream overcurrent protection device from the main overcurrent protection
device is the one that clears the arc flash event.
As an example in Figure 2, the breaker Main CB is the main protective device for the PANEL bus.
Figure 2
Figure 3 shows the arc flash table of results for an arc flash event on location PANEL. The option Main
device fail to operate, use upstream devices has been checked and a maximum arcing time of 2
seconds is applied.
Figure 3
The reported protective device is shown as MDP-CB, which is the upstream overcurrent protection
device as seen in the circuit diagram in Figure 4. When the main device Main CB failed to operate, it
trips MDP-CB at about 15 seconds based on the Time-Current Characteristics plot on Figure 4.
Technical Guide
Since the trip time is more than the maximum arcing time set for the study, the Incident Energy is
calculated based on the Trip/Delay Time of 2 seconds. On the PPE Level column, it is marked as *N22
and the name of the main overcurrent protection device Main CB. The code *N22 is explained as
“Main Device failed, use upstream device”.
Figure 4
In comparing the trip curves between the main and upstream protective devices, it can be visualized
that the upstream overcurrent protection device is much slower to clear the arcing current than the
main overcurrent protection device. Without the option “Main device failed to operate, use
upstream device” applied, the incident energy (at location PANEL) would have been 0.28 cal/cm2
since the arcing current would have been cleared by the main device (MAIN CB) at a faster time of
0.019 seconds as shown in Figure 5. With this option applied in this case study, the resulting incident
energy showed a higher value (30 cal/c m2) which revealed a very large discrepancy. This may lead to
serious safety concerns if this scenario is to occur since workers will be under-protected.
Figure 5
Technical Guide
Obviously, maintenance of protective devices is extremely critical to the safety of the system and its
workers and is the key to prevent reaching the state of having unintentional delay in the protective
devices due to failure of operation. The situation may be improbable when proper maintenance is
established. However, having the option to analyze the study for possible failure of these devices
would further help companies design their safety program or improve the protection scheme of the
power system to prevent catastrophic consequences.