Earth Pins on Power Tools

[Burnsy]

I was of the belief that all power tools used on building sites must have earth pins (unlike some of the cheaper tools). I don't really know why, but figured it was safer - maybe someone can explain.

Anyway, I had an interesting experience today while planing the top of my soon to be workbench, planed my cord . I know this is pretty common with power tools and I have nicked cords before but this time I cut right through the active. I isolated the tool straight away and was amazed to see that my RCD switches had not tripped. Cut the cord to replace the plug and I find the plane has no earth wire, even though it has a factory fitted sealed three pin plug. It is not a cheap plane but a quality Hitachi one that is about 20 years old.

Can someone explain, the imporatnce of earth pins and their relationship to RCD's if there is one and has anyone else ever found this on a Hitachi or other piece of equipment, I am not really happy about it but figure that you can't do much 20 years on?

[echnidna]

Its obviously double insulated which are not earthed

[Burnsy]

Its obviously double insulated which are not earthed

Don't know what that means but does it stop it from tripping the RCD? Do you need an earth pin for an RCD to trip?

[Barry_White]

If it as Bob say if it is double insulated it doesn't need to be earthed. This usually indicated by a Double Square symbol somewhere on the tool. Like one square inside another square.

[masher]

Its obviously double insulated which are not earthed

THen why would it have an earth pin on the plug?

[ian]

Don't know what that means but does it stop it from tripping the RCD? Do you need an earth pin for an RCD to trip?

double insulated means the tool doesn't need an earth wire, so it doesn't have one
Wikipedia says:
The basic requirement is that no single failure can result in dangerous voltage becoming exposed so that it might cause an electric shock and that this is achieved without relying on an earthed metal casing. This is usually achieved at least in part by having two layers of insulating material surrounding live parts or by using reinforced insulation.

look for this symbol on the tool

I'm pretty sure you shouldn't earth a double insulated tool.


as to the RCD.
again refering to wikipedia
A residual current device (RCD), or residual current circuit breaker (RCCB), is an electrical wiring device that disconnects a circuit whenever it detects that the flow of current is not balanced between the phase ("hot") conductor and the neutral conductor. The presumption is that such an imbalance may represent current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit

based on this the RCD's didn't trip because the planner isn't earthed (so there was no current leakage to the earth wire) and you didn't come close enough to the live wire (so there wasn't any leakage to earth through you).
The current in the active and neutral stayed "balanced" even with the cord cut.


ian

[Burnsy]

A residual current device (RCD), or residual current circuit breaker (RCCB), is an electrical wiring device that disconnects a circuit whenever it detects that the flow of current is not balanced between the phase ("hot") conductor and the neutral conductor. The presumption is that such an imbalance may represent current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit

based on this the RCD's didn't trip because the planner isn't earthed (so there was no current leakage to the earth wire) and you didn't come close enough to the live wire (so there wasn't any leakage to earth through you).
The current in the active and neutral stayed "balanced" even with the cord cut.

Based on that, I am right in assuming it would be safer to cut completely through the cord on a power tool with an earth pin as this should trip a RCD device (that is required to be used on construction sites) and not leave the exposed wires live and this is probably why tools used on site must be earthed - if what I have been told in this regard is correct. I understand the economics behind a company using three pin plugs on a double insulated tool if this is what they usually buy they would get them cheaper than getting a supplier to also supply two pin plugs. What worries me though is the assumption that I made in that it has a three pin plug, therefore it is earthed, probably the same assumption a worksafe inspector on a site might make. Surely this would be in breach of some standard?

Just checked the manual and you are right, it is double insulated.

[lubbing5cherubs]

I have no idea here but shouldn't cut that wire cause it to trip. I know mine tripped just with kids putting a knife in a toaster and cutting a cord with scissors(Not all my kids thankfully) but both time the earth leakage tripped out. I would get an electrician it to check it faulty. Your life depends on them tripping when you cut a wire I think it should of went Boonk quick. That my 2 cents
bye Toni

[martink]

What has been said about double insulated is true however the mail reason for the earth pin (and earth wire in the cord) is for when it's cut right thru - for example with a circular saw. The short carries thru the saw blade and straight to earth. In this case your circuit breaker/fuse/etc would blow yet you would most probably live.

[Burnsy]

I have no idea here but shouldn't cut that wire cause it to trip. I know mine tripped just with kids putting a knife in a toaster and cutting a cord with scissors(Not all my kids thankfully) but both time the earth leakage tripped out. I would get an electrician it to check it faulty. Your life depends on them tripping when you cut a wire I think it should of went Boonk quick. That my 2 cents
bye Toni

What has been said about double insulated is true however the mail reason for the earth pin (and earth wire in the cord) is for when it's cut right thru - for example with a circular saw. The short carries thru the saw blade and straight to earth. In this case your circuit breaker/fuse/etc would blow yet you would most probably live.

Yes this was my understanding as well, that if you cut through an active and kept going into the earth it would trip the RCD, I only cut the active so therefore no tripping of the RCD (will never know if I would have cut the earth as there was not one). I just find it wrong that a tool that is commonly used on construction sites had the earth pin and it was not in use.

Do all double insulated tools have no earth or are some still earthed? All my Makita tools have three pins and I would be surprised if none of them were double insulated if this is the standard that is being reached by the cheaper tool companies.

[ian]

Yes this was my understanding as well, that if you cut through an active and kept going into the earth it would trip the RCD, I only cut the active so therefore no tripping of the RCD (will never know if I would have cut the earth as there was not one). I just find it wrong that a tool that is commonly used on construction sites had the earth pin and it was not in use.

Do all double insulated tools have no earth or are some still earthed? All my Makita tools have three pins and I would be surprised if none of them were double insulated if this is the standard that is being reached by the cheaper tool companies.

In the past I have come across power points that were shielded.
The earth pin disengaged the shield, but if your plug didn't have an earth pin, you were stuffed.
That might be a reason for using a three pin plug
also in my experience, three pin plugs are harder to dislodge from a power point than two pin ones — this might be a reason

Based on that, I am right in assuming it would be safer to cut completely through the cord on a power tool with an earth pin as this should trip a RCD device (that is required to be used on construction sites) and not leave the exposed wires live and this is probably why tools used on site must be earthed - if what I have been told in this regard is correct. I understand the economics behind a company using three pin plugs on a double insulated tool if this is what they usually buy they would get them cheaper than getting a supplier to also supply two pin plugs. What worries me though is the assumption that I made in that it has a three pin plug, therefore it is earthed, probably the same assumption a worksafe inspector on a site might make. Surely this would be in breach of some standard?

I'm not an electrician, so I can't comment on what would be safer if you intend cutting a live wire

But in regards to the RCD the idea behind the device is to cut the power momentarily before you or someone else is electrocuted. As you get close to the exposed live wire a microcurrent flows through the air and through you to earth, the RCD detects the imbalance in the current flow between the active and neutral, assumes there's an easth leakage somewhere in the circuit and cuts the power — ideally just before you grab the live wire.
The example of the kids sticking a knife into a toaster is the same effect, just before the knife makes contact with the live wire the small current earthing flowing creats an inbalance between active and neutral and bang the circuit is cut.


ian

[rrich]

I won't go into the RCD thing but like the ones here (GFI) they require current disappearing from either the hot or neutral (return) leg of the circuit to trip.

On the earth pin issue. There probably was a requirement for an earth pin on all job site tools regardless of double insulation or not. It is also quite possible that the requirement was written without a specific requirement that the earth pin be connected to the conductive shell of the power tool.

[Honorary Bloke]

If the tool is (was) double-insulated, there is no reason I can think of for it to have had an earth pin.

Has nothing to do with cheaper tools. Most Festools (hand tools, that is) are supplied without earth pins as they are double-insulated. Same for Porter-Cable, DeWalt, Bosch, etc.

[Bob38S]

Certainly not my field but I was told many years ago that the reason for the earth pin on double insulated tools was as previously stated to allow their use in shielded power points, also that the pin was initially kept as it was cheaper to simply use existing plugs which were easily available - this consequently changed as we now have many which only have the 2 pins.

Interesting discussion re the "safety switches" above.

Regards,
Bob

[floobyduster]

A couple of pennies worth
1. If you had cut the neutral as well you should have tripped your fuse/circuit breaker.
2. The need to have tools earthed on building sites probably related to the time before they were double insulated and/or the manditory use of RCDs
3. I can remember when over 45 years ago the use of isolating transformers were used to provide protection. (As the output side was not related to earth you could only get a shock if you touched both phase and neutral - you could not get a shock if you touched the phase and earth.)
4. Remember - cut wood, not power cables