CA1123942A - Multipole ground fault circuit interrupter with trip level adjustment - Google Patents

Abstract

13 48,018 ABSTRACT OF THE DISCLOSURE

Multipole ground fault circuit interrupter appar-atus with a compensation network for maintaining a constant trip level despite less than all power poles being ener-gized. The compensation network has matched resistors respectively connected to each power pole and an additional resistor connected to the common point of the matched resistors in a branch passing through the differential current transformer core from the load side to the supply side and connected to the neutral conductor, producing a desensitizing current when less than all poles are ener-gized.

Description

3~3L2 4~,01~ :

MULTIPOIE G~O~D F~ULT CIRC~JIT
I~TEE~UPT13R WITI~ T~IP T~VEL ADJ~TP~NT
` ~L~N
This i~entlon relates ~o ground ~ault; in~e~
rupter~ ~or personnel pro~eetion aI~d partlcularly to s~ch in~errupters adapt~d for use in mllltipole applicakion~.
In ~en~ik e~ al U~,S. Pa~erlt 3,999~1C)3, iss~ed Dec~m~er 21, 197&, i~ disclo~ed a multipole grouncl ~ault circui~ breaker with inte~pting c~pability in the event of a gro~d ~ul~ ~rom any o~e OI two or more line con-ductors or power poles o~ ternating curren~ electrical distribution system that ~ncludes~ ln addltion to ~he line conductor3J a neu~ral cond~ctor that i~ gr~unded proximat~
the alternating current source~ l~eatures o~ such apparatus are also disclosed in M~senci~ U~S~, Patent 4~01~9169 M~r¢h 29, 1977, and aanadia~ ~plication Ser~al Noi. 2465961 filed March 2, 1976 by Zal~r~ewski e~ al,.~, a~signed ~o the present assignee O Th~ expsrience to the present ~ith such apparatus has been generally quit~3 sati~actory. :However~
~here i~ an aspect o~ its marmer of opera~io~ that can c~use undeslrable opera~ion under some circl~nstance~
Ground faul~ inter~pters for p~rsonnel protec~
~ion are required ~y industry standards to main~ain a trlp

2 48,018 level that is nominally 5 milliamperes, that is the current level produced by a ground fault which causes interruption is to be in any given device equal to 5 milliamperes plus or minus l mil].iampere. Achievement of this ~rip level i5 readily within the capability of the apparatùs described in the foregoing patents and application. With both poles of a two pole ground fault circuit breake:r energized, which is the normally expected case, a satisfactory trip level within the standard is produced. But, with only one of the two poles energized, it has been found by experience that there is a tendency for the trip level to be reduced to a lower than desirable level. That is, the amount of current causing interruption can therefore be sQ low as to cause -~ nuisance trippi.ng of the apparatus. It is primarily with respect to the problem of providing an easy means of trip level adjustment in multipole ground fault lnterrupters that the present invention is directed.
The trip level is primarily determined by the characteristics of the sensing amplifier and trip circuit of the device. A given amplifier and trip circuit is subject to possible variation in performance depending on how it is biased. For example, a sense amplifier and trip circuit that has been found to be very useful is that disclosed in Engel et al ~Patent 3,852,642, `December 3, 1974, which has an advantage oE being operable on a half wave rectified supply, as wel.l. as a full~wave rectified supply. In the latter case, encountered in multi-pole applications, the off-time of the amplifier :is reduced to a minimum.
3o It is found that if a unit is designed and made

3 48,01~
to have a trip level of S.0 ma. on a full-wave supply~ that same unit will exhibit a reduced trip level on a half-wave supply. The amplifier for a 2-pole GE' breaker is set at 5.0 ma. assuming both poles are on. If the unusual occwrs in ~he use of such a unit and the user forces the handle for one pole on and the other pole off, then the ampli.fier is seeing a half-wave rectified supply tha-t reduces the trip level, typically about O.S ma. That is, if a unit is tested and found to operate at a trip level in the low part of the acceptab:le range, say 4.2 ma. with both poles ener-gized, it could have a trip level of 3.7 ma., outside the desired range, with only one pole energized. This inven-tion is aimed to correct that situation.
In part, the present invention utilizes princi-~,~, ;~s~
ples discussed in Misenick et alIPatent 4,056,837,~November ~-- l, 1977 which shows how a resistive circuit branch connected between line and neutral conductors on opposite sides of a differential current transformer of a GFI influences the effective trip level of the device.
Another situation has been found in multi-pole GFI applications which is to be improved. Where the elec-trical system is three phase, the phase relation of the line voltages is such as to reduce the trip level. So if a given unit is made to exhibit a 5.0 ma. ~r trip level on single phase operation from two poles, its trip level is reduced if applied to a three phase system, even ;.f all poles of the three phase system are on. An aspect of the invention is therefore to extend the utility of GFI's for permitting application of a single unit to either single or multi-phase applications.

~o~
S~<I~T OF ~ ~ INVE _ In aGcordance with the present i~ntiQn, ~he circuitry of a multipole ground fault interrupter~ ~ich may be generally of the type as d~sc~ibed in the a~ore~
mentioned U~S~ Patents 3?999,103 and k,O15,169 and in Canadian application Serlal ~o. 246~961 or Qf any other : similar type of multipole ground ~au:Lt interrup~er~ i~
modi~ied by the addit~.on o~ a compen~ation network to prevent trip le~el reduction due to ~elec~ive pole oper~
a~ion~ that is with less tharl all poles anergized. The compen~a~ion netw~rk comprises matched impedance~, pre~
~erably resistors7 con~ected respecti~el~ to each of the lnput hot lines and wlth the impedances ha~ng their other ~e~inals ~on~ected ~o a con~on p~i~ fxom which a third i~pedanceJ pre~erably also a resistor, ~ con-nected in a circuit branc~ ex~end~ng ~hrough the core of the di~erential curren~ ~ra~former in a dlrection ~om the load side to the supply side ~d terml~a~ing by con-nection with the neu~ral conductor.
This added compensation network permit~
achie~ment o~ khe desir~d 5 milli~mpere plus or minus 1 milliampere trip l~vel under conditions o~ a single pole being energi~ed o~ the multipole apparatus ~y permitting the trip level to be set by selection o~ the third impe-dance~ This impedance will deter~ine the amoun~ o~ current flowing through the circu-.Lt branch in ~he tran~former core that ~ends to desensitize the apparatu~ and pro~ide some-wha~ o~ an ele~ation of trip level. Xn the e~ent that all poles are ener~i~ed~ ln a ~ingle phase s~stem then 3Q the compensation networ~ has no effect on operat~on, in-cl~ding no e~ect on originall~intended trip level~

~3 4gJ01~

Furkhermore~ the compensakion networ~ can be u~ed ~o pro~ide a ~ontlnuing function in ~hree phase applications by pro~iding a d~sensitlzlng current even with all poles on~
~ t ean be seen ~he compens~tion network of the present inventiGn has unique fea~ures and capa~Iitie3 as compared with the appara~us desc~ibed in khe aforementioned U~S. patent ~,056,~37 on trip level ad~u~tment.
Further aspec~s of the arrangeme~t~ in accordan~e with the present i~en~ion a~d ~heir operatio~ w~ll be found by re~erence to the dra~in~ and the ensuing ~e~crip~
~ionO

Figure 1 i~ a circui~ sch~ma~ic diagram o~ an embodime~t of the present i~entio~ in a single phase applica~ion9 and Figure 2 is a partial schematic diag~am o~ a throe phase appliea~ion o~ the in~ent~on, P~EFERRED EMBODIMENrS OF THE INVENTION
~0 Referri~g to Figure 1~ there are show~ two power conductors Ll and L2 and a neutral conductor N
of a three~

c 2~ f~2 6 ~l~,018 wire AC power distribution system. Each of the line conduc-tors Ll and L2 carry equal and opposite voltages such as in a 120/240 volts single phase electrical distribution system The neutral conductor N is grounded proximate the supply.
Within the dashed line box 8 are generally included those elements of the apparatus that are contained within the unit normally made and sold as a ground fault interrupter circuit breaker which has terminals 1 through 6 provided - for connection with the distribution system conductors that are external to the unit. The apparatus could be contained in an electrical wall receptacle or a portable ground fault interrupter unit as well as in a circuit breaker intended to be mounted in the load center o~ a distribut.ion system.
The conductors L1, L2 and N pass through a mag-netic core 10 of a differential current transformer prefer-ably as single ~urn primary windings. The core also has a ~: secondary sensing winding 12 wound on the core in normally a plurality of turns. The leads of the sensing winding 12 go to a sensing amplifier and trip circuit 14. The ampli-fier and trip circuit 14 responds to signals developed by the sensing winding 12 as a result o~ current imbalances in the primary conductors that are indicative of a ground fault that may be harmful, such as a line to ground resis-tive path as indicated at 9. If such an imbalance occurs, the trip circuit l.4 causes the opening of circuit brealcer mechanisms 17 and 18 and resulting protection for an indivi-dual who may he in the ground path 9 from the line conductor.
Solenoid trip coils 15 and 16 are individually provided in association with breaker contacts 17 and 18 with one end of each connected to respective line conductors Ll and L2.

7 4~,01~

The mechanisms are preferably connected together (dashed lin~ 19) for concurren~ operakion of the bre~;er con~acts on each of the line conductvræO ~he other end of the trip coils lS and 16 are con~ecked to respective ~ener diodes 21 and 22 which are poled in the same direction betwee~
the respective lines Ll and L2 and a common lead to the trip circuit 14~ Okher aspects wi~h respect ko khe supp~
o~ power ko the trip circuit, including lead 26, zener ; diode 2~ and capacitor 30~ and the manner of operation and chara~ter o~ suita~le trip circuits are ~iscussed in re~erred-~o U~S~ Patents ~015,169 and 39~2~642~
In accordance with pre~erred practice~ the apparatus comprises a grounded neutral transformer core 32 with a primary winding ~4 that is connecked through respective di~erenk valued resistors Rl and R2 to ~he line conductors Ll and L2 and who~e seco~dary is ~he neutral conductor N~ The purpose and na~ure o~ operation o~ the grounded neutral ~ransformer and its connection are descrlbed more ~ully in the a~orementioned Canadlan applicat~on Serial No. 246~961~
The appara~us also include~ a test circuit branch 36 connected bet~een a point on Ll on the load side o~ the d~fferential trans~ormer 10 and a poi~t on the line s~de o~ the pr~mary wi~ding 34 of the g~ounded neutral kransformer~ This branch includes a manually-operable s~tch 3~ and a resistor 40 ~or establishing a curre~t ~low to check the operabllit~ o~ both t~e dif ~erential current trarlsformer 10 and the grounded neutral trans~ormer 32~ ~urther description o~ this portion of the apparatus is in U~S~ Paten~ ~ov 37930,1~7~issued Decem~er 30~ 1975, to M~sencikO
The porkions o~ the apparatus thus ~ar desc~ibed j~!

4;2 ~:!Ol~

are merely exemplary of a pre~erred embodiment in ~hich the present in~ention is used. The i~entio~ pertains most directly to a compensation ~et~rk 50 that has individual matched resistors R3 and R~ respecti~ely con~
nected to each o~ the line conductors Ll and L2 on the su~ply side of the ~rans~o~mer lOo me resistors R3 and R~ are connected at t~eir other ends to a co~mon po~nt 51~ The reason for hav~g these resistors or other current deYeloping impedances in the ind~vid~al circuit br~nches to the lîne conduc~or~ is because at some time a user mlght deenergize one pole o~ the multi~pol~e breaker and produce a reduction in trip le~el o~ trip circuit 14 tha~ has a charact~ristic difference in per-formance when its bias changes from ~ull-wa~e rectified, as produced by both poles being on~ to hal~-wave recti-fied9 with one pole o~
The res~stors ~3 and R~ are clo~ely matched~
~uch as within abou~ 1~, to ensure tha~ the~ do not modify ~he trip level o~ the apparatus when both power poles Ll and L2 are energizedO ~3 and R4 ara also o~
apprec~able lmpedance val~e9 such as greatar than 100~000 ohms each7 to maintain an ~dequa~e isola~ion between Ll and L2~ Prom the common poin~ o~ R3 and N~ there is connected anokher re~istor R5 who~e other end is con-nected to the neutr~l conductor N by a conductor 52 that extends around the core to the supply side of ~he tran3 ~o~merO
~ lth both poles Ll and L2 energi~ed, that is carrying their normal~y~intended current and voltage~
there is zero current through the Compensa~iQn net~ork because the currents developed by R3 and R4 are equal and opposite and c~lcel each other at common point 51 ~ence9 when both .~ .,.~ .~.;
A ~ i 9 48,018 poles are energized, the compensation network 50 produces no effect on the operation of the apparatus.
When only one of the power poles Ll or L2 is energized, there is a net current flow through the compensa-tion network and a desensitizlng current ID flows through conductor 52 that reduces the sensitivity of the apparatus.
The desensitizing current ID equals the voltage on the energized pole divided by the sum of the resistor R3 or R4 connected to that pole plus RS; i.e., 10Volta e (Ll) Volta~e (L2).
R3--g--F~j or R~-~ RS ~~~~
This amount of current desensitizes the apparatus because it requires that an additional ground fault current of that amount, in addition to the fault current otherwise required, be drawn to cause the apparatus to be actuated.
~ or example, typical ac-tual values encountered in practice are:

Trip level with both poles energized 5 ma. + 1 Trip level with only 1 pole energized, without compensation 4.5 ma. + 1 20Resistor values for compensa-tion in a 120/240 v. system:
R3 120k ohms.
R4 120k ohms.
R5 lOk ohms.
The invention is also applicable to ground fault interrupters of more than one phase. For example, applica-tion of the .invention has been made to three phase appara tus having nom:inal voltages s-wch as 120/208 volts.
Figure 2 shows a representative connection of a 3 ground fault circuit breaker 8, as in Figure 1, to a 3-phase wye distribution system. Here the compensation . - .

lQ ~8,018 : network 50 in the uni.t 8 will supply a densi.tizing current ID in branch 52 throughout operation when either or both Ll and L2 are on. In this arrangemcnt the third phase lead L3 goes to another breaker and does not influence operation of the unit 8.
As a result of the practice of the invention in accordance with the foregoing description, it will be seen that it provides a way to avoi.d overly-sens-.itizing the apparatus as a result of only one pole of the multipole system being energized while fully preserving the capabil-ity o~ the apparatus when all poles are energized, in either a single phase system or a three-phase system.
Thus, there is achieved through a relatively simple exped-ient not unduly complicating the apparatus or its ability to be made economically and compactly, a system that pro-vides a uniorm trip level under a variety of conditions.

Claims (5)

11 48,018 CLAIMS:

1. A multipole ground fault circuit interrupter comprising:
a differential transformer core, a plurality of primary windings on said core including at least two line conductors L1 and L2 and a neutral conductor for connection with the conductors of an AC electrical distribution system;
a secondary sensing winding on said core for sensing current unbalance between said primary windings;
means responsive to a predetermined sensed signal on said sensing winding to open said line conductors; and means for providing a desensitizing current when one of said line conductors L1 and L2 is energized and the other is not energized, said desensitizing current being absent when both said line conductors L1 and L2 equally and oppositely are energized

2. A two pole ground fault circuit interrupter in accordance with claim 1 wherein:
said means for providing a desensitizing current comprises a compensation network connected between said line conductors and said neutral conductor, said compensa-tion network including two impedances respectively con-nected to each of said line conductors at one of their 12 48,018 terminals, the other terminals thereof having a common connection, said two impedances being of matched character-istics, said compensation network also including an addi-tional impedance connected between said common point and said neutral conductor in a circuit branch which connects to said neutral conductor.

3. A Multipole ground fault circuit interrupter in accordance with claim 2 wherein:
each of said impedances of said compensation network is a resistor.

4. A multipole ground fault circuit interrupter in accordance with claim 1 wherein:
said distribution system is a three-wire, single phase distribution system.

5. A multipole ground fault circuit interrupter in accordance with claim 1 wherein said distribution system is a three phase voltage distribution system and said compensation network provides a densitizing current both when a single pole is on and when two poles are on.