How to execute protective grounding of electric units
Let's consider general information about grounding of electric units. The serious danger is constituted by touch to not current carrying metal parts which appeared energized owing to isolation damage.
If the touch to current carrying parts can be preyoduprezhdeno barrier or their arrangement at neyodostupny height, then the touch to parts to the netokoveyodushchy, for example, equipment body, is inevitable at any operation. Moreover, in some cases this touch is normal working operation. The mechanic is obliged to concern, for example, periodically ruyoky the electric motor body to check to the touch extent of heating of its parts. Working with the portable electric tool is in dlitelyyony contact with its body.
The worker servicing the machine which electric motor is installed on one bed or on one shaft with it it is long Proyoboy of isolation at such type of electric equipment neizyobezhno is connected via the machine with the electric motor body, etc. involves transition of tension to the engine body, to the tool and the machine therefore the rayobotnik appears under the influence of electric current.
The unexpectedness of this phenomenon and unpreparedness of the worker to it often lead to accident.
Reduction or elimination of danger upon transition of tension to the body and not current carrying konstruktivyony parts of the electric equipment is reached by one of the following measures: protective grounding, zayoshchitny shutdown, the covering of not current carrying parts isolation or their production from the isolating mayoterial, application of the isolating supports, ponizheyoniy tension, etc.
As the most reliable measure of protection of the person from the pereyokhod of tension on not current carrying parts serves the zazemyoleniye — the metallic compound with the earth of netokoveduyoshchy metal parts of the electrical unit, koyotory, being located near current carrying parts, can appear with them in contact.
Body of elektriyochesky machines, transformers, rheostats, kontrolleyor, metal casings of switches, plugs, frameworks of boards, metal covers of cables, the body of couplings, drives of the electric equipment, the farm, column and other not current carrying parts of electrical units which can appear accidentally energized are subject to protective grounding.
Depending on tension and the system of the electrosupplying network with isolated or the deadly earthed neutral of transformers (generators) protective grounding carry out differently (fig. 1).
Protective grounding in installations with the isolated neutral (fig. 1, a) the power transformer (generayotor) is carried out connection with the earth of netokoveduyoshchy parts of installations which can appear under the nayopryazheniye at violation of isolation and to which vozyomozhno touch of people.
Creating the metallic compound of big conductivity between the body and the earth, reach that the current passing through the body of the person included payorallelno to this connection becomes harmless.
In network to deaf grounding of the neutral (fig. 1, b) the siyolovy transformer (generator) for grounding not current carrying parts of installations connect to the zazemlenyony zero wire. Grounding serves in such installations for the reliable and fast automatic otyoklyucheniye of installation at faults to frame of povrezhyodenny sites of network.
At fault to frame of the electric motor proyoizoydt the short circuit between the damaged phase and the zero grounded wire, in the chain there will be the short-circuit current, and the damaged equipment of the avyotomaticheska will be switched-off from network as predoyokhranitel will burn down or the automatic machine will be switched-off.
For fast and reliable shutdown of the povrezhdenyony site the short-circuit current has to prevyyoshat not less than three times rated current of the fusible insert of the safety lock or by one and a half times current of the setting of the next automatic cut.Top