Need for earthing and double insulation


earthing

Earthing is the act of connecting the metal casing of the appliance to earth via a wired connection to the bare ground. Earthing wires are usually have a green and yellow bands around them. Why do you need earthing? Consider this scenario:

  • The live wire is frayed and touched the metal casing of the appliance. (Another phrase for this is: The metal casing of the appliance becomes live.)
  • The appliance do not have an earthing cable.
  • YOU touched the metal casing of the appliance.
  • Your body would have completed the circuit $\rightarrow$ electricity will pass through your body.
  • The current is not high enough to trigger the breaking of fuse in the power plug.
  • You get electrocuted.

If you have earthing (connecting the metal casing to the ground), the current will have two paths to take to complete the circuit:

  1. The low resistance earthing wire
  2. Your body (Note that your body have very high electrical resistance (orders of magnitude larger) when compared to the earthing wire)

From the theory for current flows in parallel circuits, the current through the low resistance earthing wire will be orders of magnitude higher than the current flowing through your body (which will be negligible). In effect, the situation can be summarised as “the current flowing through the low resistance earthing wire instead of your body”.

Hence, the earthing will divert the current into the earth by providing an alternate path to the large current flow via the earth wire, rather than through the user’s body.

Double Insulation

There are some appliances which do NOT have an earth wire. They have another way to protect the user: double insulation. Double insulation protects the user of the appliance from an electrical shock by preventing any possibility of the external casing becoming live (the live wire can not touch the casing even if wires inside become loose), thus eliminating the need for an earth connection. The two layers of insulation are:

  • First insulation: Insulating electrical cable from the internal component of the appliance.
  • Second insulation: Insulating internal metal part which could become live from the external casing.

Note: If the external casing is plastic, there’s no way the external casing can become live.

You can identify which appliances have an earth wire by checking the mains plug. If it is a 3-pin plug (all three pins are made of metal), it would have an earthing wire.

 


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9 thoughts on “Need for earthing and double insulation”

  1. “You can identify which appliances have an earth wire by checking the mains plug. If it is a 3-pin plug (all three pins are made of metal), it would have an earthing wire.”

    In the UK this is not the case. All mains sockets are three pin so all appliances have to have three pin plugs, whether or not they have an earth wire. The only exceptions I’m aware of are electric shavers which are supplied with a two pin plug which requires an adapter to be used with a three pin socket. There are two pin sockets specifically for shavers which are often found in hotel rooms, so an adapter is not needed. There are clearly labelled “Shavers ONLY.”

    Reply
  2. So why do some appliances have stickers stating “Double insulated do not earth”. Does that mean there is no need to earth or is the reason to not earth such as a danger if earthed?

    Reply
    • Kent.
      This is an interesting question. And one I was looking to get answered here myself.
      In equipment with double insulation and a metal outer casing, which contrary to what people say does exist I have come across in the past a Marantz Amplifier and a Yamaha DX7 Synthesizer like this.
      The outer metal case is insulated from the inner chassis by a plastic chassis or the inner chassis is plastic.
      Unless you modify the equipment there will be only a 2 wire cable supplied with no wiring going to the chassis so you cannot earth it anyway
      As long as the outer chassis is not connected to the internal electronics then earthing it will not effect the performance of the equipment.
      The only reason I can think where this might be the case ( no pun intended) is that the metal chassis is screened to the signal ground lines and creating a faraday cage to stop noise.
      Earthing it in this case would introduce noise into the equipment.
      If something is double insulated it doesn’t need an earth its safe to not be earthed.
      Manufacturers have to prove its safe before they can get electrical safety approvals on their equipment. If the equipment is approved then there will be a label close to the mains input showing the power requirements and stating how it complied to electrical safety.
      In the case of the Marantz amplifier I fitted a 3pin mains socket to replace the original 2 pin and fitted an earth stud in the chassis and took the earth wire directly to that. I then star bonded from there to all the other outer metal parts such as the lid and control panel.
      And in that I’ve probably answered my own question

      Reply
  3. This sentence:
    “The current will obviously travel through the low resistance earthing wire instead of your body. ”
    is just blatantly wrong. Anyone with the slightest knowledge of electrical theory can confirm that this is not how parallel circuits work. Electricity does not “choose the path of least resistance”. Please do not spread misinformation like this.

    My understanding, though I am not a professional electrician or electronic engineer, is that the purpose of grounding metal casings is so that if the live connection contacts the case, then a short will occur in the mains, through the device long before any human can touch the device. This short will usually blow a fuse, isolating the circuit from the mains or otherwise tripping the circuit breakers in the household or whatnot.

    Reply
    • In the case of a fault (live wire touching the grounded metal casing), the current will travel to the ground via the earthing wire. The resistance of the earthing wire is negligible as compared to your body’s electrical resistance. This will result in the current through your body being negligible. In this scenario, the electricity has effectively chosen to travel through the path of least resistance. (It is just the law of big numbers).

      With the current traveling through the earthing wire, the low resistance of the earthing wire will cause a high current flow. The high current flow will cause a fuse to be blown or the ECLB to be tripped.

      Nevertheless, I recognized your concern and have amended the post.

      Reply
    • Typically an equipment has a switch that is used to turn it on. If the wire touched the case at location that is on the other side of the switch it cannot short the connection before the person touches the equipment. Also there may be a problem that occurs only in certain situations.

      Reply
    • How is the current not high enough to melt the fuse while the resistance of the circuit is too low and current is infinite.
      My doubt is in this part:
      The current is not high enough to trigger the breaking of fuse in the power plug

      Reply
      • The line
        “The current is not high enough to trigger the breaking of fuse in the power plug”
        is meant to reflect a situation where the equipment is not earthed and the current is going through a person rather than an earth wire. because the equipment is not earthed.

        If you have a live chassis situation on an unearthed piece of equipment there is nothing to take the current to ground so the power will stay on the chassis. until you touch it

        A persons body has a high resistance compared to a wire.
        if that person becomes part of the circuit they will draw current as a resistive load following ohms law between the live chassis and ground.
        in this situation the person touching the equipment may draw 100s of mAs which is plenty enough to kill them but nowhere near enough to blow a 3A fuse
        Its kind of how “The Electric Chair” works

        Reply

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