IEEE Std 142-2007 "Grounding....Power Systems" has two equations [ 2.3a & 2.3b] that help define the thermal boundary for wire insulation protection based on I^2t, the initial wire temp, final temp, and wire xsect area. The equation 2.3a is I^2t/A = 0.0297ln[(Max Temp + 234)/(Initial Temp +...
I'm investigating using the subject PLC to control a wastewater treatment process. I'm planning on 20 digital I/O and 16 analog I/O and I'm trying to estimate the memory needed. The AB selection guide gives a memory of 4000 bytes needed per controller task, but I'm not clear on what constitutes...
I'm in a disagreement with an Engineer with a major electrical equipment manufacturer over the basics of a thyristor controlled soft starter. I am trying to calculate the voltage drop profile on the supply system during starting.
The Engineer's analysis shows the initial starting voltage at...
A local company has approached my client with an offer to install a "device" that increases power factor to reduce KWH consumption thereby reducing utility cost. They claim that PF improvement lowers current, which is true, but that it also lowers KWH usage by the same percentage which is not...
I'm interested in getting the names of manufacturers of Supervisory Control and Data Acquisition (SCADA) equipment/systems for utility type electrical power distribution systems.
Can anyone help?
Several manufacturers recommend that a three phase delta-delta transformer, with the 240V secondary center tapped to serve lighting or other 120V loads, should have the 120 volt load limited to only 5% of the kva rating of the transformer because of the resulting loading caused by circulating...
NEC 250.6 covers objectionable currents on grounding conductors, but gives no guidance as to what level of current is objectionable. Soares "Book on Grounding" states that it's any current that prevents the grounding conductor from maintaining earth potential at each grounded device. This makes...
Does anyone know of a tech literature source that describes a way to calculate the zero sequence impedance of three phase circuits in grounded steel conduit? The process for overhead circuits is straight forward, following the methods in "Symmetrical Components" by Wagner&Evans, however it...
I'm familiar with the general purpose and use of these type transformers, but would be interested if anyone knows of any resource that provides a reasonably rigorous technical discussion of how the transformer reacts to various unbalanced conditions.
The 2002 NEC "Handbook" provides an example of how to size a transformer and select overcurrent device ratings for a 4160v supply to a 480v 100hp fire pump and 1.5hp jockey pump. It's covered in NEC695.5(A)&(B) and is a straight forward process to select a 150kva transformer.
Where...
I'm designing for 3-100HP electric fire pumps. Per NFPA 409 two of the pumps are to be adequate for the system demand with the 3rd pump as a standby. It's my interpretation that the transformer would be sized for 125% of the full load of two of the pumps, not three.(NEC 695.5(A)) The pumps are...
We have two transformers at a hospital that were recently tested by the manufacturer. Both are 5 years old and are 12.47kv-480v delta/wye. the 5000kva unit's Percent Impedance test revealed an average of 9.5 ohms and a calculated X/R of 1.6. Nameplate is 6.07% and typical X/R is closer to 9. The...
We have a 5000KVA 12.47kv-480v delta/wye transformer in a hospital that was recently tested. The Percent Impedance test indicated an average of 9.5% with a calculated X/R = 1.6. The nameplate %IZ is 6.07%. The typical X/R for a power transformer of this size is closer to 9.
Another transformer...
NEC 250.24 (B) requires a grounded conductor in "under 1000 volt" services. Does the NEC require a grounded conductor for over 1000 volt, and if so, what article specifically addresses this?
