I'm working on a study for a client interested in replacing existing MCCs with arc resistant MCCs. All of the duration rated MCCs I've found have a duration rating of only 100 ms. For this particular system, the available fault current is less than 20 kA. With the short time trip delay on the...
We have a large hydrogenerator excitation system in which the PPT (power potential transformer) that feeds the exciter is fed from a a fused tap off the generator IPB. The PPT secondary conductors are then fed to the exciter cabinet via cables in tray. The AC breaker they land on has no...
We have a retrofit 15 kV generator breaker with a GSU side TRV capacitor rated 200 nF. After installation we discovered that the capacitor nameplate indicates that the capacitor does NOT have an internal discharge resistor. This is the first time we've seen a medium voltage capacitor that did...
We have an installation in which an the switchgear and PTs were replaced for an older hydro generator but the regulator was not. It was not discovered until after the system was installed that the regulators (Westinghouse BJ-30) specifically calls for an open delta PT circuit. The regulating PTs...
We are struggling to find specific requirements for equipment that must be installed in a Class II Division 2 Group F location. From the NEC description it would appear that NEMA 12 is sufficient but that doesn't give us a lot of confidence that we're 100% covered. Is there any clear guidance...
We're reviewing existing settings for a loss of excitation protection scheme and have found that the offset and mho circles were developed based on unsaturated reactances rather that saturated values. This doesn't make sense to us since system voltage should be near nominal. Would this be done...
We are working on an upgrade for an industrial customer with some very old portions of their system. The original distribution was installed in the 30s or 40s and was entirely 240V delta. A few years ago they changed the main service to 480V to feed some newer loads but retained the 240V system...
We have a number of old 4160V Allis Chalmers motors that we plan to replace with new motors. We'll need to adapt the frame but can't find any information on the existing frame dimensions. The motors are 100 Hp and have a frame size stamped on the nameplate of 23BS8. It also states that they...
I have found several posts discussing testing of MCCBs with various opinions on size and required testing but not many have provided backing references. We have had a difficult time testing breakers smaller than 100A and have opted to perform primary injection acceptance testing only on...
With regards to ABB FT-1, 10 pole test switches, we have for years used the part number 129A514G01 which has four PT poles and six current poles. That specific configuration has one of the PT poles on one side of the current switches and the other three on the other side. ABB has a different...
We've seen a recent DC motor failure on a large hydro turbine shutoff valve actuator. The DC motor is 125VDC, 6 Hp, long-shunt compounded. The controls for it use a simple reversing contactor to reverse the armature connections. After the failure we found a few issues with wires sizes but the...
We have a small oil pump with an emergency backup power supply that when the normal supply comes back there is the potential for a very fast restart. Effectively, the backup contactor opens and the normal source contactor shuts. The motor is a 2 Hp, 3-phase, 480V induction motor. We...
The question has come up as to how to size the feeder cables to a motor circuit with a soft starter per the NEC. It would seem that article 430.122 would apply and that, with a bypass circuit, the feeder needs to be sized by 125% of the soft start rating or the motor FLA, whichever is higher...
We have an industrial facility that is losing critical motor loads on outside line faults. It appears to be a classic case of voltage sag that is dropping bus voltage to 50-70% for as much as 20 cycles. We are working on relay improvements to provide faster clearing but that won't change the...
A post a few years ago discussed the calculations associated with zig-zag grounding transformers. In it one user posted the following:
"You might need to reconsider an almost entirely reactive impedance for the grounding. It is generally considered a bit of a no-no to have reactance grounded...
We have a pump with an ~25' shaft running at variable speed up to 229 RPM and power up to about 450 Hp. It had some initial high vibration but after two years of operation it is now vibrating to the point of tripping. Extensive vibration and modal analysis has been done and the conclusion is a...
We have a small Pelton Wheel type hydroelectric project with two needle valves. The needle valves have an internal hydraulic actuator and one of them has a compromised seal and is leaking oil. That is a big problem in the environmentally sensitive area. I've seen needle valves with a...
I am a bit confused by the 2014 NEC with regards to VFD feeders and I'm hoping light can be shed. The feeder conductors must be sized for 125% of the DRIVE rating or 125% of the MOTOR FLA if a bypass device is used and the MOTOR FLA is higher than the drive rating. So far this makes sense...
I have a control panel in which the UL508 panel builder grounded the CPT secondary neutral via a screw on the back panel. The back panel is grounded. It seems to me to be a bad practice considering there is a ground bus within the enclosure and the builder just got lazy but I don't know if it...
We have a grease lubricated journal bearing on a vertical shaft of an axial flow pump that demonstrates an unusual temperature cycle. The grease is injected every several hours into six equally spaced, helical grease grooves by a centralized system at high pressure. Each time grease is...
