Document QkGdOq8d1zgq7pOw7EBy7gXZ7

Theie 3-Prong Plugs Assure Equipment Grounding Newer grounding reeeptode, left, permits use of regulor 2 ly plugged equipment Is not rulod out If this receptacle is used.' 1 prong plug or moling grounding cop. This meons conventionol- Grounding plug-receptocle to right locks this flexibility feature^ Flexible Pigtail Connettions Are OK It Properly Made e-.'.inm.ml or_o_u_n__.ii.io^ loud ll brought out |uustt obovon thbo pplluuQs.. Into 'ccoplncte center. Ballery clip, right, mokei pood O'ound StqtueqexwT*wT'g,vro^d wire, uleft, tildes over rmnoelue rpeiinn sthteantt csrcrsreowwes connection to circuit conduit. Atioch ground before plugging |-X to Ground Portable Electrical Equipment Good grounding of portable equipment it a safety must in the maintenance program. Here are four of the approved methods along with practical lips showing how each is best used. Rec* ommended minimum values for resistance to ground are listed By H B .WHITAKIR, Sr Aitoehto fhcttkcl fegineer Underwriters Laboratories Joe Thcrs xnt three practical means of grounding portable equipment: (1) ap proved grounding plug with one prong tying'frame of portable tool to. circuit conduit (2) grounding conductor run along with the'circuit wires and con necting to an approved mullV-prong plug; grounding prong ilea between ground wire and frame of portable equipment (3) separate flexible wire or strap- may be insulated, or bare, con necting directly to a suitable ground. When properly- applied, any of these methods will prevent equipment frames becoming live to ground. , Groundlng'ot Outlet. When you boil it down, the biggest problem In ground ing portable equipment is completing the gruumJing circuit at the supply out let. The present widely used parallelslot receptacles and parallel-blade plugs are 2-poie devices. These devices don't lend themselves to automatic grounding when the attachment plugs are inserted. Many attempts have been made to lick this problem through use of adap ters and pigtail connections on the grounding wire. Hut human element always has to be depended on to estab lish the grounding circuit. Multi-Prong Plugs. A major step for ward was recently made when the grounding multi-prong plug and re ceptacle, above left, was developed. This receptacle handles existing 2-polc parallei-blode-type plugs, or 3-pole at tachment plugs having a round or halfround grounding blade. Since recep tacle, will take exisiing-parailet ltJodo plugs, it does nut present a problem on equipment with 2-pule plugs. There nre additional advantages to using this newer-type receptacle plug: grounding Is automatically handled, plus the fact that the inherent design rules mil use of the 3-prong-lype plugs where oil poles may be alive (as is met within 3-phase^ i*i Vwhere such a lug can be alipped under portable tools). $ >*. a^machine screw. De cautious with 3-prong receptacles]? -Multi-prong plug has the basic ud- and plugs that may bo used to feed 3-2 (* - vantage over pigtail connections in that phase loads and- 3-wlre Edison systems;^ 'grounding is automatic each time the as well as 2-wire systems with equlp-^ attachment plug is inserted. On tho ment grounding. This variety of pos- vr i-; cither .hand, there is one major disad- sible uses means we can occasionally^ * ...vantage with many of the older multi- expect accidents when portable toolsb intended to hove a grounded frame aron plugged into a 3-phase receptacle. , When a manufacturer supplies >$ multi-pole attachment plug on portable^ equipment, it is no guarantee that the# user will supply a suitable receptacle.^ j.';rprong pluga end receptacles; they im/,> .'mediately rule out existing 2-polo plugs -on' present equipment. -'^Another weakness in the pigtail pic^T-;lyre It that the proper sequence of maVTK7|hg^connections is left'entirely in the ^"'Operator'* hands. The ground connec- Often the user replaces the multi-prong^ tion should be mode before the tool is with a.2-pole plug. And In ao doing he'fc may get the' ground conductor mixed up,-3 leaving the tool frame ollye rather than$ grounded. I] Pigtail Connections, Various arrange* y menta of the pigtail Idea are recognixed > as suitable grounding methods. One M K`energised. *!- Where grounding wire is fjari'ef a flexible cord assembly, moke } v^gwunding connection in. the tool with r^v -machine screw. This simplifies rei.-^f.onneetlngthe ground wire If and when i' ^fW.iupplycord Is replaced: Croundtng the plug-end-jack type of connector, S)'.' connection' hnnds all exposed non-cur- facing page. Here the ground wire, separate from the power plug, slides on -, a grounding terminal permanently al-.j (ached to the receptacle. Another method uses a battery clip- /fi-r"- eorry*"R ports of the tool. J.V-l'.'.Craiinrf twVr for portable equipment ty'-bis No. 18 or Jft Awg when fuses or tit breakers are set-within 20 amps, tjy.y*? 14 wire where the overcur- This Is one solution where portable & equipment will be used at various out-^ i<V ^ev^e u ret between 20 and 30 l`- i^oiperej. ^rr,Pending to Eorlh. Yntir grounding lets around the plant, assuming that *!] conduit or water-pipe connection Is si- * j mu#t have * low resistance to ways handy. Sparle-type lugs with uf>^ ^ .jniurc that enough current will flow turned ends are often selected sd grounding terminals. Hern you cn'0 f|Vg?n:*n RMldentarground to trip the -^Voven;u.^,u device. Low resistance elso prntmrl at any point on metal rocewaf.sj V.r^vn* ,^,0< w>th current flowing In the grounding circuit, connected equipment ia never at an appreciable abovo-ground potential. Here are some safe rules to follow: For a 120-v circuit, protected by a 30omp (use or breaker, the grounding re sistance should be less than 3.6 ohms. It's a good rule to keep the resistance of weter-pipe grounds at about 0.25 ohms. Resistance of the connection to' earth, at the plant grounding source, Is often the biggest resistance in the complete grounding circuit. Where driven grounds are used the resistance to ground should not be greeter then 25 ohms. Actually if you look into it, this resistance is not low enough to blow a 30-amp fuse on a 120-v circuit. But It does give * meas ure of protection against accidental crosses between primary and secondary of a distribution transformer. STEP-SAVING WIRE CHECK ____ tJQOinon/ms- Wires or eotUe being cAvatatf ** illHP Ji |Hj Save lime when "ringing out" wiring in cable or conduit by first hanging separate resistors of different ohmsge across leads at for end. Tag wires No. 1, 2, 3, etc, corresponding to site resistor used. Your ohmmeter will pick them out at this end. L JT Fitzpatrick, Jefferson City, Mo PUNT OPERATION AND MAINTENANCE SECTION POWElJ 1933 PLANT OPERATION ANO MAINTENANCE SECTION