I was at a site today (new sewer pump station) that will be supplied from a 3 phase delta. Upon checking the meter socket for power i read 240v to ground on a phase, 0 volts to ground on b phase and 240v to ground on c ph. i get 240 v phase to phase. i dont have much experience on this type of system but i thought b phase was the high leg? i'm a little confused does anyone have an answer as to what i should be reading?

thanks
Kelley

Your readings are fine.

You have a 3-phase, 3-wire corner grounded delta system, there is no high leg.

A high leg can be found with 3-phase, 4-wire delta systems.

[This message has been edited by iwire (edited 11-10-2004).]

so 240 to ground on a and c phase is ok. thank you iwire

Kelley

 
For 240V delta, regardless of system grounding method, be aware that it is a serious misapplication to use less than 250V fuses or other than 240V circuit breakers. 120- or 120/240V-rated circuit breakers are not intended to be used with 3-wire delta electric services. [See UL white book.]

Congratulations! You have found the ONE TIME you fuse/switch the "neutral!"

John,
You can't use a fuse in this grounded conductor unless the fuse is used for motor overload protection. You treat this grounded conductor just like any other white wire.
Don

By Bjarney:
____
For 240V delta, regardless of system grounding method, be aware that it is a serious misapplication to use less than 250V fuses or other than 240V circuit breakers. 120- or 120/240V-rated circuit breakers are not intended to be used with 3-wire delta electric services. [See UL white book.]
______

Please explain more.

resq- You're quite right- the only times I've encountered this type of power, it was used to power well pumps, and the control voltage came off a transformer.

The UL "White Book" says, under "Circuit breakers, moldes case..." that:
"breakers rated 125/250....are suitable for use....where the voltage to ground does not exceed 125v."
I believe that the "White Book" is available at UL's web site.

also with delta breaker you can not use the slash on the label like 120/240 volts it must be straght 240 volt becaue of line to ground is pretty high btw it will stated in nec but i cant rember where it is excatally located

merci, marc

240.85 Applications. A circuit breaker with a straight voltage rating, such as 240V or 480V, shall be permitted to be applied in a circuit in which the nominal voltage between any two conductors does not exceed the circuit breaker’s voltage rating. A two-pole circuit breaker shall not be used for protecting a 3-phase, corner-grounded delta circuit unless the circuit breaker is marked 1–3 to indicate such suitability.

A circuit breaker with a slash rating, such as 120/240V or 480Y/277V, shall be permitted to be applied in a solidly grounded circuit where the nominal voltage of any conductor to ground does not exceed the lower of the two values of the circuit breaker’s voltage rating and the nominal voltage between any two conductors does not exceed the higher value of the circuit breaker’s voltage rating.

FPN: Proper application of molded case circuit breakers on 3-phase systems, other than solidly grounded wye, particularly on corner grounded delta systems, considers the circuit breakers’ individual pole-interrupting capability.

 
For the 2003 UL White Book in pdf, see Joe T's first link at https://www.electrical-contractor.net/ubb/Forum1/HTML/003428.html {~2MB}

Maint'guy, as I understand the situation, some ground faults on delta systems can require one breaker pole or one fuse to operate, where the voltage across the opening pole may be at line-to-line potential, causing failure in a underrated overcurrent device.

In a 120/240V 3-wire or 208Y/120 system, this condition cannot occur, so "slash-rated" breakers are not a misapplication. The ‘single-pole’ rating for ungrounded, corner-grounded or 4-wire delta 240 and 480V systems are subject to the “no slash” mandate.

The fuse side of this for 240V-delta systems—150V fuses are a misapplication, but OK for 208Y/120V service—as are 300V fuses OK for a 480Y/277V systems, but not in a 480V-delta system.

The short version of all this is that underrated overcurrent devices may delflagrate trying to interrupt a fault or overload.




[This message has been edited by Bjarney (edited 11-14-2004).]