Need sound advice please

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discusted electrician

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Sorry if this is lengthy but it is neccessary. I have several problems but will tackle them one at a time (all electrical and not emotional or personal).I have a 1500 kva 3phase xmfr that feeds a 4000A MDP. Their are 10 sets of parallel feeders from the xmfr to the MDP. The primary feeders are 500kcmil w/ a 350kcmil grounding conductor (neutral) with NO grounded conductor in each conduit (PVC). The MDP is not connected to a ground grid system but is bonded to building steel w/ a 3/0 copper. The NEC 2005 Table 250.122 shows that the neutral is undersized (4000A) and 310.4 also states the grounded conductor " must be the same size in circular mill". Is it mandatory that all service entrance conductors (parallel feeds) have a grounded conductor included and is the neutral undersized according to my understanding? Also, I have been experiencing severe problems with equipment that rely upon electronic boards that are constantly frying ( motors with variable speed drive VSD). Since much of the metal raceway that runs throughout the facilty do not have grounded circuits included in the branch feeders, but rely solely upon the metal raceway as grounding, could this be causing transient voltages to be applied back on to the PC board and blowing circuits. The facility is 30 years old and with 8 overhead cranes running along a rail system 12 hours a day, much of the counduit has vibrated loose from the connector, coupling, walll etc. thus loosing it's continuity and ground fault protection. Any help is greatly appreciated and thank-you.
 
Your description has me a bit confused. I think you are using ?grounded? and ?grounding? backwards. You say you don?t have a ?grounded? conductor, then talk about the neutral being undersized. Those two terms are generally considered synonymous. The ?grounding? conductor, or more precisely the ?equipment grounding conductor,? or EGC, is the one that provided the ground fault path, and that might not be needed if the raceway is metallic.

You ask about the neutral being undersized. But more importantly the phase conductors (ungrounded) are undersized. Ten sets of 500 MCM are only good for 3800 amps, not for 4000. So you need to either run another set, or make sure the overcurrent protection is set no higher than 3800 amps.

Is this a service transformer? It is owned by the utility? Have you confused the grounded and grounding conductor terms with the ?Grounding Electrode Conductor,? the one that connects the MDP to planet Earth?
 
the xmfr is owned by the utility company. ten parallel feeds with just A-phase, B-phase, C-phase and neutral with No ground.All ten conduits are underground in PVC conduit. Mdp is not connected to a ground grid system but is bonded to building steel.
 
discusted electrician said:
the xmfr is owned by the utility company. ten parallel feeds with just A-phase, B-phase, C-phase and neutral with No ground.All ten conduits are underground in PVC conduit. Mdp is not connected to a ground grid system but is bonded to building steel. their are currently no egc pulled in with the 10 parallel feeds and conduits are 4" PVC.
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Sounds to me like the service conductors are fine since this is an old building and you were permitted to go to the next size overcurrent device in the past. The serving electric utility would only permit three phase (ungrounded) conductors and one neutral (grounded) conductor in each set of service conductors. An equipment grounding conductor would not be permitted since everything on the line side of the service equipment is bonded to the neutral.

After leaving the main service equipment, a neutral is not required to be taken with the feeders or branch circuits unless it is needed, this would be an engineering decision. A grounding conductor is required for all feeders and circuits. Those grounding conductors are permitted to be metallic conduit. :smile:
 
DE, Sounds like a good project to set up for the next 'shut down', tighten up

all that loose stuff, while your at it, maybe run EGC's. I have heard ' electronic

guys' say they use the ground (egc) for part of their circuit, if that is the case

at your place and the system is as bad as you say, maybe that's frying the

boards ? Just a guess!
 
30 years ago, the university hospital I was responsible for had problems with "something" tripping the laboratory computer server into shutdown-reset. I placed my oscilloscope on the line and waited for several hours until the "transient" occurred. The administrator bought a "isolation" transformer for this equipment.

Just a thought for you to consider.
 
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