Fusing Lateral Tap-Offs in an Ungrounded Wye System
Fusing Lateral Tap-Offs in an Ungrounded Wye System
(OP)
What are the advantages and disadvantages of installing a fuse(s) on a single phase tap-off from a three phase line to prevent the recloser in a substation from unnecessarily operating for faults on the single phase tapoff in an ungrounded wye system?
Because it is an ungrounded wye system, there is no neutral wire and for "single phase" customers the utility needs to run a second phase. Therefore, one disadvantage I can think of off the bat is for a single phase tap-off you need 2 fuses. If the system was a grounded system with a neutral wire, you would only require 1 fuse at the tap-off. Are there are any more advantages/disadvantages?
Because it is an ungrounded wye system, there is no neutral wire and for "single phase" customers the utility needs to run a second phase. Therefore, one disadvantage I can think of off the bat is for a single phase tap-off you need 2 fuses. If the system was a grounded system with a neutral wire, you would only require 1 fuse at the tap-off. Are there are any more advantages/disadvantages?






RE: Fusing Lateral Tap-Offs in an Ungrounded Wye System
Single phase phase L to N is not same as single phase between two 'phases'. The L-L voltage is 1.73 times L-N voltage. So you can't substitute L-N supply with a L-L supply.
In theory, you can still provide a L-N supply wiht one fuse even if the Wye system is ungrounded. It is not necessary to ground a system to derive a neutral!
Why would you even have a ungrounded wye system is a totally different matter and not the subject of you question. Fusing is not even a criteria to compare a grounded and an ungrounded system.
RE: Fusing Lateral Tap-Offs in an Ungrounded Wye System
RE: Fusing Lateral Tap-Offs in an Ungrounded Wye System
For ANSI regions, a checklist of sorts, is ANSI/IEEE C62.92.4-1991 …Application of Neutral Grounding...Distribution
Assuming distribution voltages, the basic philosophy of overcurrent-device coordination [e.g., “fuse saving” versus “trip saving”] does not change markedly comparing ungrounded to grounded systems.
OTOH, insulation coordination is treated quite differently, as implied by rbulsara. Ungrounded systems can have potentially serious ground-overvoltage problems.
RE: Fusing Lateral Tap-Offs in an Ungrounded Wye System
The 3phase 4wire power distribution system would require one fuse in the line conductor only.
RE: Fusing Lateral Tap-Offs in an Ungrounded Wye System
Chaps, I need to clarify my first post. Your second post showed up before my first post, and I had yet to see your second post.
Your description seems more like it is a C62.92.4 three-wire unigrounded-wye system and not an ungrounded-wye system. In a typical utility setting, the transfomer-secondary neutral point [XO] serving the distribution circuit is often solidly grounded through a substation ground grid, but a neutral conductor is not routed with phase conductors [as in a multi-grounded system.]
As you stated, with a three-wire unigrounded-wye system, loads are served phase-to-phase. Also, earth return is relied upon to sense ground faults, and it is typical for ground-overcurrent relaying to be set at relatively low values.
RE: Fusing Lateral Tap-Offs in an Ungrounded Wye System
RE: Fusing Lateral Tap-Offs in an Ungrounded Wye System
RE: Fusing Lateral Tap-Offs in an Ungrounded Wye System
RE: Fusing Lateral Tap-Offs in an Ungrounded Wye System
jghrist — I agree. At 50 amperes (primary) coordination is not possible with larger fuses. It is a limitation in the design.