- Plastic enclosures
- Metal shields
- Dust and moisture protection
- type of instalation
- Dimensions and number of modules
- Features of the design of the shields
The electrical box body seems to be such an insignificant detail that many people buy the first one they come across without even trying to understand the difference between the models. In this article, we will explain the difference between metal and plastic, different degrees of protection and types of electrical panels..
Electrical installations of consumers with voltages up to 1000 V can form rather complex circuits, for switching and protection of which group shields are arranged, sometimes containing several dozen devices. This, as well as the method of installation, the place of installation and operation, the area of responsibility for the electrical facilities at the facility and some other factors explain such a wide variety of housings for the main distribution and group panels. Ultimately, any of them are made from either plastic or metal..
The choice between plastic and metal is a compromise between high natural durability and resistance to mechanical stress during operation. However, over the past 5-7 years, thanks to precise calculations and new methods of material processing, the line between plastic and metal has somewhat blurred. At least the products of time-tested brands have an equally long service life, regardless of the material of manufacture. In the case of using low-quality Turkish or Chinese electrical engineering, when choosing a shield in a damp basement, it is better to pay attention to plastic options.
There are also more significant differences: plastic shields do not require mandatory grounding (although they have a group of protective conductors on the block), therefore, when laying two-wire circuits, only them can be used. This applies in particular to ungrounded lighting lines, control circuits, special structures with equipotential bonding protection and circuits with IT earthing systems. Conventionally, some metal shields with an inner dielectric shell (plastic false walls, painting with non-conductive enamel) can also be classified as plastic. In most household energy supply projects, preference is given to modular plastic enclosures. This is perfectly acceptable if it is not part of a bad and inappropriate habit..
Metal cases have a number of special properties, the main of which is high strength. The higher the equipment class in terms of permissible current, the more force is applied when switching equipment, not every material can withstand this. Most plastic three-phase boxes can withstand no more than a dozen D100 circuit breakers, with such loads the case must be made of metal. So, plastic shields should not be filled with modular equipment with a rating higher than 63 A, and the possible characteristics of releases for them are B, C, A. In addition, do not forget that packing a loop of wires (with a 30 cm remainder) of a three-phase input is not only extremely tiresome. The proximity of wires causes hum, poor air flow and overheating. On the other hand, metal models from Hager or ABB are quite spacious and ergonomic, they can pack 15-20 three-wire outgoing lines.
There are also special cases when the body of the shield must be made of metal. This applies to distribution points of the TN-C system and switchboards, on the case and door of which light-signaling equipment or measuring instruments are installed. The door, in the presence of mounted electrical equipment on it, must be connected to the body with a flexible cable. That, in turn, is connected to an isolated PE-conductor bus. It is also better to assemble relay devices and control circuits in metal shields, using the case as a common zero or GND.
Dust and moisture protection
Requirements for the IP indicator – dust and moisture resistance of the shell – practically do not differ for metal and plastic cases. The degree of protection is determined by the operating conditions of the most sensitive filling elements.
Most of the common plastic shields are IP43, metal ones are IP32 and IP21. This is quite enough for living rooms, but such ideal conditions are not always found..
Outdoor shields must have IP54 and not lower, and installed without a splash guard (on supports, masts) – IP66.
If an isolated lockable room is allocated for the distribution point, the value of the switchboard housing is purely nominal. Often, modular devices are generally attached to the wall with an open canopy on a DIN rail, although for convenience it is customary to arrange them in enclosures with a minimum degree of security. The same applies to the filling of complete switchgears in panel housings..
type of instalation
All enclosures are divided according to two main installation methods: overhead and built-in. The latter are distinguished by the presence of a flange on the front panel covering the technological joint. The choice in favor of this or that method is made for aesthetic reasons and depending on the load of the cable network.
It makes sense to install built-in shields if the number of outgoing lines is at least half the maximum number of modules for the shield. Otherwise, it is very difficult to pass such a loop of wires through the side walls. It is better to purchase a shield with a removable back wall, which can be easily installed on top of a bundle of wires coming out of the wall, it does not even have to be ordered.
Dimensions and number of modules
It is generally accepted that the number of dashboard modules should be a multiple of twelve, but this is not a mandatory requirement.
Small-sized shields have from two to twelve modules, but their number is necessarily even. Such buildings are used when arranging lighting boards, as well as storey and group ones. In some cases, small shields combine the function of junction boxes.
The middle panels – for 16, 18 and 24 modules – are used as main switchgear or floor panels. Such shields can have up to two tiers to accommodate modules.
Large panels for 36 and 48 modules have 3 and 4 tiers, respectively. They are usually used where it is necessary to combine all electrics with a low current in one case; in some models, internal screening partitions are provided for this.
Among other things, a large shield can also be a wall cabinet – with one or two doors and free placement of elements inside. This is usually how the driveway risers are arranged..
Features of the design of the shields
If the shield is installed before the metering device, its body must have ears for sealing with a cord and tight cable glands. Otherwise, the supply line will not be accepted by the energy sales inspector.
For organizing the cables, the side walls often have holes for mounting patch brackets that support the ribbon cable. Such devices for securing the cable can also be located between the rows, which is very convenient for installation.
Another important detail is the gap between the rail and the back of the case. It should be 2-3 times the thickness of the wire from which the connecting jumpers are made.