Earthing Double insulation

What if we have an exposed conductor that is not connected to earth (such as for equipment with a two-wire ac cord), or if earthing is itself not an acceptable level of protection for that particular type of equipment as per safety regulations Then, besides the layer of basic insulation, we need another insulating layer (with identical dielectric withstand capability). This is called supplementary insulation. Together these two layers (basic + supplementary) are said to constitute double insulation. We could also use a single layer of insulation, with dielectric withstand properties equivalent to double insulation (i.e. 3000 V ac or 4242 V dc). That would then be called reinforced insulation. So for example, if the equipment is by design, meant only for a two-wire ac cord, we would need two layers of approved insulators (or... 

Line-to-earth capacitors are called Y-capacitors. Since Y-caps are critical in terms of having the potential to cause electrocution if they fail, approved Y-caps are typically impulse-tested up to 5 kV peak. Note that Y-caps used anywhere on the primary side (in off-line applications) must always be safety approved. Depending on the location in the power supply, we may even need two Y-caps in series (basically corresponding to double insulation). However, sometimes we can also find Y-caps placed between the secondary ground and earth/enclosure (for EMI suppression purposes). In this position, it is usually acceptable to use any ordinary 500 V ac rated capacitor (unapproved). 

The device may be grounded for safety reasons (safety class I, as shown in Figure 10.35, maximum resistance in the protective earth conductor between power plug and chassis 0.2 Q), or double insulated (safety class II). 

Earth bonding tests apply a substantial test current, typically about 25 A, down the earth pin of the plug top to an earth probe, which should be connected to any exposed metalwork on the portable appliance being tested. The PAT Tester will then calculate the resistance of the earth bond and either give an actual reading or indicate pass or fail. A satisfactory result for this test would typically be a reading of less than 0.1 n. The earth bond test is, of course, not required for double-insulated portable appliances because there will be no earthed metalwork. 

A flash test tests the insulation resistance at a higher voltage than the 500 V test described above. The flash test uses 1.5 kV for Class 1 portable appliances, that is, earthed appliances, and 3kV for Class 2 appliances, which are double insulated. The test establishes that the insulation will remain satisfactory under more stringent conditions but must be used with caution, since it may overstress the insulation and will damage electronic equipment. A satisfactory result for this test would typically be less than 3 mA. 

The principle is that the live conductors are covered by two discrete layers of insulation. Each layer would provide adequate insulation in itself but together they ensure little likelihood of danger arising from insulation failure. This arrangement avoids the need for an earth wire. Double insulation is particularly suitable for portable equipment such as drills. However, safety depends on the insulation remaining in sound condition and the equipment must be properly constructed, used and maintained. 

Apparatus of the double-insulated or all-insulated types need not be earthed provided it complies with the Electricity Regulations 1908 (Portable Apparatus Exemption) Order 1968. This order, in exempting such apparatus from the earthing requirements, makes it a condition that the apparatus is clearly marked (i) with maker s name, model and type number (ii) with an indication that it has been certified, by an approved body, as being apparatus conforming to the approved standard for such apparatus. It is also a condition that the insulation of the apparatus is maintained so as to prevent danger. ... 

Portable tools are used extensively on sites and maintaining them and their connecting cables in good repair is a critical factor of their safe use. Electrocution occurs when the body acts as the conductor from a power line and earth, often because the earth connection on the tool has broken. All portable tools must be securely earthed or be of double insulated construction and the plug on the lead must be correctly fused. Unfortunately it is frequently difficult to keep track of every item, so reliance has to be placed on the person using them. 

To remove the need for earthing on some portable power tools, double insulation is used. Double insulation employs two independent layers of insulation over the live conductors, each layer alone being adequate to insulate the electrical equipment safely. Since such tools are not protected by an earth, they must be inspected and maintained regularly and must be discarded if damaged. 

Precautions shall be taken, either by earthing or by other suitable means, for example, double insulation, use of safe voltages and earth-free non-conducting environments, where a conductor, other than a circuit conductor, could become charged as a result of either the use of, or a fault in, a system. 

It is important to ensure that all portable electrical equipment is regularly inspected and adequately maintained to minimise the risk of danger to the user. Equipment of the double-insulated or all-insulated types (class II), to the relevant BS , has no provision for earthing and is not earthed. Such equipment should be marked with the s5anbol [ ]. There is no symbol for class I equipment. 

Portable apparatus may be of Class I construction, in which case the metalwork is earthed by a protective conductor which is a separate core in the flexible supply cable, or it may be of Class II construction, having no protective conductor terminal as its metalwork is not earthed. The essential safety feature of Class II apparatus is that the basic insulation is supplemented by additional insulation to provide a further safety barrier and danger arises only if both insulating layers fail. There are two types all insulated where the supplementary insulation is a plastic case, and double insulated where there is a metal case but live parts are separated from it by two layers of insulation. It is not advisable to use enclosed ventilated Class II apparatus in wet environments, because moisture may penetrate and provide a conductive film between the touchable surfaces and internal live conductors. 

If Class II distribution apparatus or accessories are used. Regulation 471-09-02 requires the protective conductor to be available at each wiring point to allow for the replacement of the Class II by Class I equipment. It is preferable for the apparatus and accessories to be of the all-insulated rather than the double-insulated type to avoid the possibility of exposed conductive parts making fortuitous contact with earthed metalwork. 

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