Why this resistor?

[Shel-e-bean]

Does the UK system use a "neutral"? IE, a main conductor bonded to ground (earth)?

 

[AndrewT]

Hi,
our "neutral" varies depending on which method the supplier uses to connect the main incomer. We use 2pole + earth for our domestic systems. Commercial use the same 2pole + earth derived from the 3phase or 3phase +neutral (4 wire) or 3phase +N +E (5 wire)

There are a multiplicity (TT, TN, and lots of others) some just connect the "earth" to "neutral" in the premises others connect the earth to neutral farther upstream. I think some (one?) keeps the earth actually in the earth (ground).

Don't ask me for details, I have only a sketchy recall of the alternatives. See this http://en.wikipedia.org/wiki/Earthing_system

 

[cpemma]

Does the UK system use a "neutral"? IE, a main conductor bonded to ground (earth)?

Neutral is nominally at ground potential, but once in the premises Neutral should be kept separate and a 'Safety Earth' provided, usually connected to the supply cable sheath from the main fuse. In addition, copper central heating/water pipework is connected to the same spot, plus for bathrooms extra rules apply.

Supplementary Bonding

In a bathroom there are many non-electrical metallic components such as metal baths and basins, supply pipes to bath and basin taps, metal wastepipes, radiators, central heating pipework and so on, all of which could become dangerous if they were to come into contact with a live electrical conductor. To ensure that such an occurrence would blow a fuse or trip a circuit breaker in the consumer unit, Wiring Regulations specify that all these metal components must be connected one to another by an earth conductor which itself is connected to a terminal on the earthing block in the consumer unit. This is known as supplementary bonding and is required for bathrooms even when there is no electrical equipment installed in the room and even though the water and gas pipes are bonded to the consumer’s earth terminal near the consumer unit. When a shower or heater is fitted in a bathroom, that too must be supplementary bonded by connecting its metalwork, such as the casing, to the non-electrical pipework even though the appliance is connected to the earthing conductor in the supply cable.

http://floti.bell.ac.uk/engapps/domestic.htm

 

[Shel-e-bean]

Hi,
our "neutral" varies depending on which method the supplier uses to connect the main incomer. We use 2pole + earth for our domestic systems.
<snip>
See this http://en.wikipedia.org/wiki/Earthing_system

Thanks for the link. I've looked for something like this at Wiki before, but I never saw this page. One thing I'm not clear on.. Is there a current carrying conductor in a residence (or in appliances) that is at earth potential? The diagrams at Wiki don't show the conductors "in use". In Canada, residences use the "TN-C-S earthing system", and most appliances connect between L1 (or L2) and N, giving them about 110-120 V. Large appliances like stoves and Driers use all three wires, with the heating elements connected across L1 and L2 for 240 V, while motors and timers get 120v. Other things, like large portable tools, might use L1 and L2 and a ground, no Neutral required. Whether a separate ground is used on portable appliances depends on the appliance itself.

 

[AndrewT]

Hi,
Cpemma may have answered, but here goes.

We use live and neutral in our single phase "domestic" system with a protective "earth" (PE).

So all our three pin sockets have L + N + E going back to the distribution board.

Most equipment uses all three wires to ensure safety.
Some equipment is double insulated (classII) and can safely dispense with the earth wire. However our 3pin sockets are shuttered so the double insulated plug top needs a dummy earth pin to open the shutters to allow the plug top to be inserted.

I see a comment in Wikipedia about the (non) safety of the US and Canadian systems for using 2phase (L1 + L2) into higher power equipment. Have you seen that page yet? It was written by a non Brit so no axe to grind.

 

[Shel-e-bean]

Hi,
<snip>
I see a comment in Wikipedia about the (non) safety of the US and Canadian systems for using 2phase (L1 + L2) into higher power equipment. Have you seen that page yet? It was written by a non Brit so no axe to grind.

First, thanks all for the info.

Andrew;
Re the higher power equipment, no I haven't seen that page. As long as there is a safety ground wire, I can't imagine much difference in safety between 120 and 240 V single phase. :-/
Could you find it and post a link? I've tried a few pages without luck.

 

[macboy]

I'm not an electronics tech, so my worry with adding a ground wire would be ground loops. Not even new equipment has 3 wire cords... they just use standard polarized 2 wire cords like incandescant lamps. (In the old days, you could get a shock just changing a lamp's bulb if the screw shell was hot. You could plug it in any old way. There was no upside-down). I guess the new equipment is electrically isolated from the mains. To me, the wonderment comes from the fact that tuners aren't like tape players... They need antennas. That involves grounding. My head is spinning. javascript:smilie('')
xeye

I agree. Do not add a ground connection, as it invites ground loops.
The power supply for this thing is tranformer coupled, so ground is completely unecessary. The "ground" provided by the secondary of the transformer should be floating, and connected to the chassis. Connecting this to the ground of the power line could cause more problems than it solves (which is: none).

The resistor is a safety measure, as odd as it seems. It is there to prevent the chassis from becoming much higher or lower potential than line. It will not conduct enough to cause a shock. Still, I would use a polarised plug and make sure that the line with the resistor to the chassis is on neutral. I have seen this same thing (with a polarised plug) on a brand-name receiver made around 1998, so it is not a 70's thing.

 

[AndrewT]

Hi Shel,
http://en.wikipedia.org/wiki/Talk:Mains_power_plug
Look near the bottom "T-slot......in Canada"

 

[Shel-e-bean]

Hi Shel,
http://en.wikipedia.org/wiki/Talk:Mains_power_plug
Look near the bottom "T-slot......in Canada"

Ahh.. The kitchen plug thing. That is the one thing that has had me scratching my head for years. Our old requirements for kitchen counter plugs were truly assinine. They mustn't have asked any electrician experienced in residential service about it.
For the non-Canucks, in kitchens, the counter plugs are "split", the top half of the plug gets one half of a 2 pole breaker, the bottom half of the plug gets the other half of the breaker, and they share the neutral. The outlet halves must be disconnected from each other by snipping the little bar that joins the hots.
I've had numerous calls from people who change their outlets, then can't get the breaker to hold... because they created a short across 240 volts! Sometimes, half the breaker holds and half trips! They might leave it that way for years, too. Or they keep trying to figure it out, and keep blowing the breaker. It's a wonder the breaker stays in one piece. I found one welded closed... Turning it off didn't turn it off!
Ooh ohh.. My favorite.. The "Stab-lok" panels made by FPE are built so you can plug in a 2 pole breaker across 1 phase. All other panels are laid out so that a 2 pole breaker connects across both phases. So if you plug in the kitchen counter breakers across just one phase, the neutral carries double the current. This happens all the time. Inspectors miss it on a regular basis, too. Sometimes the kid wiring the house plugs all the 240 v. breakers in wrong and has to change them because the stove, dryer, hot water tank, baseboard heat etc. is all dead. They will come and fix all the 240 loads, but leave the kitchen breakers! I could go on and on.
Yay for the new code!