The extension is the same. necessary element electrical wiring in the house, like sockets, switches and other equipment.
In the store you can find dozens of their varieties. Which one to choose and what parameters to pay attention to in the first place.
Wire length and number of sockets
Remember that the longer the carrier, the greater the voltage loss. For household appliances, the optimal length of the extension cord is from 3m to 7m.
At the same time, you can connect the load to them with a power of not more than 3.5 kW (up to 16A) at the same time. It depends on the size of the wire.
For more, socket contacts are simply not designed, no matter how thick the wire turns out to be.
By the way, many sockets for sockets are not needed for simultaneous connection a large number devices, but in order not to constantly pull out the plug, thereby protecting against loosening the contacts. 
Cross section and brand of wire
The wire cross section for connecting the load of room equipment (TV, computers, refrigerators) should be at least 0.75 mm2.
Many do not know, but all extension cords must be manufactured and comply with GOST R 51539-99 (IEC 61242-95). It is called "Extension Cords for Household and Similar Purposes on Cable Reels". You can download.
Here are the regulated wire sections for extension cords according to this standard:
Wire brand - most often PVA with PVC insulation. 
In this case, the diameter of the wire itself (not a separate core) is desirable to have from 5 mm and above. If this is not the case, the manufacturer most likely saved on something - either copper or insulation. 
The KG brand can also be used instead of PVA. It is more resistant to negative temperatures and changes in weather conditions. For street or garage use, this choice would be more correct.
PVA drag several times over concrete floor and his isolation will come to an end. KG is more resistant to such operating conditions. Other brands of cable are applicable: 
Here are the suitable wire sizes for quality carriers, and that they can be safely switched on without the risk of overheating (refrigerators, washing machines, TV, drills, grinders, heaters, etc.): 
Unfortunately, in the factory version, you will hardly find household extension cords with a cross section of 2.5 mm2 in stores. Unless it will be reels or coils of great length, from 50 meters and above. 
Therefore, such models (with wires 2.5mm2, 4mm2 and above) are often made independently.
For more powerful devices, such as inverter welding machines, there is already a completely different class of products. 
Mechanical protection
A quality model must have protection against pulling and bending the wire. 
This applies equally to the connector with a plug, and to the place where the conductors enter the extension housing. 
Never handle with twisted wires. Firstly, this will sooner or later lead to damage to the cores, and secondly, such a cable will simply heat up.
Please note that extension cords can only carry their rated load when the cable is fully unwound. On the case, sometimes special icons are applied, with the maximum current when the wire is wound and unwound.
Before buying an extension cord, look at what kind of plug it has:
If the plug is not cast, and it can be disassembled, then there is a risk of running into a dubious product. It is these forks that are easiest to make in a handicraft way.
For the production of solid ones, at least some professional equipment is needed. 
In addition, if your equipment is with a plug without a Euro connector, then an extension cord with a Euro socket is contraindicated for you. The use of euro adapters and adapters in this case increases the transition resistance at times. 
It also violates the grounding system, which can lead to electric shock. It is simply dangerous to connect some devices and tools without grounding.
The contact resistance of one contact in the new model should be no more than 0.05 Ohm. During operation, it can deteriorate to 0.1 ohm.
That is, at a rated load of 10A, only on one plug-socket contacts can you lose up to 10W in power every hour. Here is a summary of voltage and power losses in extension cords, depending on the length, material and cross section of the cores. 
And this is at normalized values. And what will happen if you poke various adapters.
Also pay attention to the location of the plug-in socket contacts. They must be made so that the plug of the device to be connected is inserted diagonally or obliquely, and not vertically.
Otherwise, the second plug cannot be connected to the adjacent socket.
Aim to choose models with built-in surge protection and protection against dust and moisture. Moisture protection is indicated by Latin letters IP. Household extension cords most often come with IP20 numbers.
They can only be used in dry and dust-free rooms, but not in the bathroom or garage. Models with IP44 protection are already splash-proof. 
Do not confuse models of surge protectors and simple carriers with a shutdown button. In the latter, a switch is built in, designed to manually turn off the power. 
It in no way protects the equipment from voltage surges and distortions. To do this, there must be at least a varistor inside the case. What it is will be discussed below.
By the way, the switches themselves are different:
It is better to choose 4 poles. It's pretty easy to tell them apart. 
The four-pole is twice as wide. It is he who breaks both the phase and zero.
With a bipolar one, you can only turn off the neutral core, and the phase will still go to sockets and appliances with the plug plugged in.
Be careful!
The insides of extensions
One of the most common types is the so-called computer extension cable or network filter. It has a built-in switch in the case, which can turn off the power to all outlets.
It is usually designed for a current of 10A (power 2.2 kW). Each socket in them has a grounding contact. What is essential for apartments with three-wire wiring (phase-zero-earth). 
The supply wire on quality models must be marked. A brand and a cross-section of the cores are stamped on it. 
Do not buy carriers where the cross section of the wires is less than 0.75 mm2 or where there are no identification marks on the wires at all.
In this case, you should not particularly trust the thickness of the cable itself. It is possible that the insulation in it will be quite thick, but the veins are very thin.
Such a wire will definitely heat up, even with minimal load. 
The most important thing in such models is that there is not just a switch inside, but an “automatic machine”, which will automatically work when the load is exceeded.
This extension cable is used to power computers, LCD and Led TVs, printers and other household appliances. 
How does it protect equipment from power surges? In order to understand this, you need to look inside the case.
To do this, loosen the screws on reverse side and remove the cover. The contact pads of the carrying sockets must be made of brass plates. 
Most of them are 0.3mm thick. Better models for high currents, have a thickness of up to 0.6 mm.
In cheap Chinese fakes, you can find not brass, but only brass-plated iron shanks-plates. You can check this case with an ordinary magnet. Non-ferrous metal will not be magnetized, but iron is easy. 
In addition to the worst current conductivity, the iron contacts will expand after the first use of a wide Euro plug, and will not return to the reverse state. Brass, on the other hand, has considerable elasticity. 
By the way, many “Soviet” plugs will not fit into such connectors due to the presence of side ground pins.
Most main element, protecting all connected equipment is a varistor. Phase and zero are approaching it from two sides. With a short-term voltage jump (about 300V), its resistance drops sharply, but the current flowing through it increases. 
Due to this, the voltage on the varistor itself stabilizes. If the jump is not short-term, but the overvoltage is constant, then it simply closes the circuit and an artificial short circuit is created inside the case.
That is, if the voltage in the sockets jumped and kept within 300V, the internal resistance of the varistor sharply decreases (down to zero), it works, creating a short circuit. 
And it is from this that the short circuit is disconnected circuit breaker protecting all connected appliances.
Higher quality products are equipped with several varistors, plus capacitors and inductors. 
In inexpensive ones, all protection ends with a varistor and an automatic or push-button fuse. 
The plates themselves are not spring-loaded here, although they are brass. And at relatively high currents, they quickly burn out.
Extension cord without switch
For more powerful loads, you can choose models without built-in switches. Protection in this case must be provided by automatic machines in the shield. 
Choose such models for a garage and a summer residence with a wire cross section of at least 2.5 mm2, since socket groups are often protected by 25A machines.
And when overloading an extension cord made with a 0.75mm2 cable, no machine in the shield will work. First of all, it is the extension cord that will begin to melt.
They also have brass contact plates inside. Sometimes they are attached to the base of non-combustible material. This is directly responsible for the fire safety of the entire block. 
One such material is polyamide. In general, for all manufacturers, the extension housing must be made of non-combustible plastic, but not all “Chinese” observe this. 
Power wires can be either soldered or welded contact welding (the best option), and simply tightened with screws in special terminal connectors. 
It is more reliable when clamping occurs with a plate, because very often these screws crush the tip of the wire, worsening the contact area and the contact itself. 
If your carrying is done in this way, then it would be more expedient to compress such wiring with a NShV tip.
Soldering at high currents and heating may fall off, but nothing will happen with welding. Even with short circuits.
In addition to the contact brass plates, do not forget to check the plug itself with a magnet. It should also be made of non-ferrous metal and not be magnetized. 
Bad extension cords
Unfortunately, most often on sale there are cheap extension cords that hide a bunch of potential dangers.
In appearance, everything seems to look civilized, and the inscriptions are reassuring. Rated currents up to 16A, voltage 250-260V! and so on.
However, it is worth revealing all the shortcomings on the face. The most painful place is the underestimated wire section. Under the “thick” insulation and molded plug, you will not see this.
But inside the case, at the points of connection to the plates, you can already compare with something. 
Next come magnetizable contact plates. This picture says a lot: 
Sometimes they are even made from tin cans, as evidenced by the pattern on the sides. Instead of brass, galvanized iron is used here. As a rule, in such models, the fork itself is made of the same material.
Even at a load of less than 1kW, the wire starts to heat up, and with a long load it may melt. All this is fraught with a fire. 
Therefore, be careful when buying and never trust only inscriptions and brand names. You can choose a high-quality extension cord for absolutely any purpose with a length from 1.5 m to 50 m.
There are models whose shell is adapted for use in conditions negative temperatures- such products are specially designed for outdoor installation, they are noticeably superior to their counterparts in terms of wear resistance. We are talking about SIP and KG, modifications with shells that provide the best protection current-carrying conductors.
The problem of using wiring in the cold
It is often forgotten that cable and wire intended for use inside buildings are not suitable for use in open spaces. Because of this, many make such a mistake as bringing internal wiring to the street if it needs to be stretched from one building to another.
Under conditions of temperature difference, the cable sheath wears out quickly. The VVG and ShVPP wires are most susceptible to this: already in a frost of 15 degrees, the vinyl insulating coating of these models becomes unusable. Vinyl hardens to such an extent that any attempt to bend or straighten the product can result in its destruction.
A little best qualities has the GGG model. Although it was originally designed for internal installation, its outdoor use is allowed in regions with mild winters, where the air temperature does not fall below 25 degrees. In central Russia and in the north, it is no longer suitable: its shell is made of PVC and tans in extreme cold. This is not surprising, because the VGG was originally developed for operation in enclosed spaces.
The best solutions for arranging street wiring
For use on outdoors special cable models have been developed - these are KG and SIP. The first modification is a soft rubber wire, partially retaining flexibility at temperatures up to 40 degrees below zero. The product is designed to fit industrial installations but it can also be used at home. The modification of KN with the HL marking has the highest plasticity indicators. This model is used in the most severe conditions.
SIP has the best indicators of mechanical strength. Its insulation is made of cross-linked polyethylene and provides maximum protection for current-carrying conductors. True, SIP is suitable only for connecting branches and individual inputs to power lines. This wire is worthy alternative uninsulated aluminum cables, which were previously used for this purpose.
In the SIP marking there are numbers indicating the number of cores. A KG cable can also have several cores - from 1 to 5. If we compare both types of wires, then KG is better suited for arranging street wiring - SIP is strictly specialized, its installation is carried out using special fittings, anchors and clamps.
