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One of the most practical substation design guides is the RUS "Design Guide for Rural Substation" publication 1724E-300. You can get it from the below link. It has specific recommendations such as minimum layer of 4" with 3000 ohm-m resistivity...

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 purchased a PowerSight data logger a few years ago to take measurements on a medium voltage rectifier. We have a number of AEMC and Dranitz loggers but the PowerSight had an option for medium voltage probes that can be placed directly on the line without PT. They are rated 15kV and come with...

This looks similar to a situation we had a few years ago on a very similar fused disconnect in a VFD panel. In that case, the fuse did blow and the drive was very close to a large, lightly loaded dry type transformer. When it blew, the voltage surge (caused by the transformer inductive kick)...

The current is different in delta/wye but not delta/delta. I'm not sure why wye/wye is not included in the allowance but here is the exact text from the code: (1) Protection by Primary Overcurrent Device. Conductors supplied by the secondary side of a single-phase transformer having a 2-wire...

The 250% protection is for the transformer only per Article 450 but the secondary conductors are covered under a different article, 240.21. In that article it is OK to use primary protection for secondary conductors if the transformer is delta/delta or single phase, single voltage. For any other...

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...

Thanks for the reply. Machines are a special exclusion because it is expected that the capacitor will discharge through them. For example, the NEC excludes self discharge on power factor caps when they are installed with the motor T-leads but not when they are installed on the high side of the...

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...

I would emphasize stevenal's suggestion, check the CT secondary grounding and make sure the neutral point is grounded at one point only. I've seen this a number of times, especially in powerhouses where there are a lot of stray magnetics. Also note, the MV cable shielding is electrostatic...

The construction is similar to an MCC without an enclosure so if you use an MCC model you'll get MCC bus gaps in the IEEE 1584 equation and VCB electrode configuration. That will yield a conservative enclosure factor since there is no enclosure to concentrate the energy but that seems...

The magnitude of fault current will be primarily a function of the wye/delta transformer zero-sequence impedance. For example, if the wye/delta is just a small PT bank the fault current will be very low since the kVA base will be very small. In any case, the wye/delta functions very similar to a...

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...

Thanks again. We appreciate your knowledge of the code. Looks like 500.8(B)(3) gives us what we needed: "Where specifically permitted in Articles 501 through 503, general-purpose equipment or equipment in general-purpose enclosures shall be permitted to be installed in Division 2 locations if...

rablex - We appreciate your response. Yes, we've reviewed 502 carefully but 110.28 does not provide as much clarity as one might hope. The NEC text (not the handbook text) explicitly states that the types outlined in 110.28 are for "other than hazardous (classified) locations." Hence our...

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...

There are quite a few 240V three phase and single phase (240V line-to-line not 138V line-to neutral or 120V for high leg delta) such as compressors, heat trace systems, etc. Lighting systems are derived from 240 to 208/120 banks or 240 to 240/120 single phase transformers. There are too many...

Yes, it's definitely ungrounded. Not high leg delta or corner grounded. We are in NEC land but this system definitely predates the codes restrictions on ungrounded systems. What baffles me most is why the previous engineering group that did the last renovations kept it ungrounded. I'd be...

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...