Power - would this be dangerous?

[iamjulian]

From a safety point of view, do you think the following sounds possible:
 

RCD plugged into a standard wall socket in the garage.  Into that, a standard plug and power cable running from the garage to an IP66 weatherproof junction box in the garden.  Inside the box, a standard "brick" type transformer (like you get with a laptop) taking the power from 240v AC to 12v DC. Then a 12v cable out to the mount.

Would the transformer be ok sealed inside the weatherproof box though given that is is not ventilated and they get fairly warm?  I am a good 20m from the nearest power point so worried that a long 12v cable would lose too many volts, even if I started with 13.8v

Thanks

[oldpink]

it will be fine, I wrap mine in a polly bag with no issues
and run off a 15m cable from the mains, I have them and my transformers sitting under a plastic tub

[Ibbo!]

I stuff all the connections and transformers into a really useful box

not had any issues

[Bizibilder]

Would it be possible to step down to 12v indoors and just run 12v DC outside?  Should be safer to run the low voltage outside.  You should get little voltage drop unless your garden is of considerable acreage!!

[iamjulian]

Not convinced I'd like to run the mains out into the garden and have it sat under a plastic tub in the snow or dew, but I admire your 'sod it, what's the worst that could happen' take on electricity, oldpink

Yes, ideally I would prefer to run the 12v from the garage, but read lots of opinions and quite a few say that unless you use a thicker (and more expensive) than standard 12v cable, you risk dropping too many volts.  I'll be about 20 meters from the power source.

[Carbon Brush]

Mains in a box with the arrangement described is fine.

Running 12V outside by 20M is asking for bother. Especially as motors, heaters, etc take current in pulses, not continuous. Making for a worse than expected voltage drop.

[Tinker1947]

I used to run 13.8v out to the mount from inside a dry room, 10m length dropped to 13.4v on the handset (its measurement) the worst thing that can happen with a mains supply to a connection box of any type is, " You Die" if it goes wrong, with a 13.8v starting voltage nothing if it goes wrong.....

[damnut]

That should be fine, only caveat is that on modern consumer units there is already an rcd on board. I would check with a qualified electrician about introducing another RCd if you are already covered.

[Carbon Brush]

The RCD used in domestic installations usually is a 30mA model. That means if the current flow in live and neutral goes out of balance by 30mA, or more, for a few milliseconds the device will trip and cut off power. The idea is that current and time are small enough to stop your heart being shocked into misbehaving. Do not try this at home!

Depending on the age of the wiring in the house, and what modifications have been made, you can have a very confusing RCD situation.

If the consumer unit is old, there may be no RCD. However, if you have had some wiring changes (new kitchen for example) then a single RCD for a particular circuit may have been fitted.

For slightly newer installations, you may have an RCD some circuits only. Or it may protect everything. It all depends on what work was done and when.

If I have it right, then a new installation requires 2 RCDs, each protecting part of the wiring. The theory being that in the event of big problem (say fire melting wiring) then at least some lights will work helping you exit the house.

No doubt someone on SGL can provide the actual regulation numbers, coverage and dates.

If you fit an RCD on the 13A socket feeding outside, then on a fault, either the socket RCD will trip, or (if fitted) the consumer unit RCD. Or maybe both. What I'm getting at is that fitting an RCD at the exit point does no harm. And is essential if the consumer unit RCD is absent.

Hope this helps.

[Sirius Starwatcher]

Just be very careful with your set up .If not done properly and you have a mishap you could negate insurance. This is without HSE getting involved. I'm not trying to scare you Just think it through logically. The rules are very complex and insurance companies will look for anything to stop paying out. As a minimum requirement I would run it past a qualified electrician first. Best of luck.

[jnb]

That would be fine. You have two levels of protection; the RCD in the garage and the IP66 junction box. All exposed elements are 12V. Heat dissipation shouldn't be an issue because the junction box will be in a cold environment, if you do find an issue with it warming up too much it will not be due to a poor layout but due to a low quality transformer

[Carbon Brush]

Something more to add to the lively debate.

The 'laptop' or 'brick' type adapters are usually switched mode power supplies. These allow much more power out in a given size/weight than the conventional transformer based power supplies. They also have the potential to be very efficient, which means there is little heat generated. Switched mode supplies do have a transformer in there. But it is quite small and there are lots of other components in there. Enough on that.

The laptop type power supplies generally get warm in use. The designers know the power supply will be left on a nice woolly carpet and covered in papers. If properly designed, then they will cope with the internal temperature build up. In practice this usually limits life of certain components 1000hours or so before they degrade, but nothing more. Serious failures (and fires) are quite rare events.

A little bit of heating in the junction box will do no harm. In fact it will help keep condensation at bay.

Hope this helps.

[Tinker1947]

Something more to add to the lively debate.

The 'laptop' or 'brick' type adapters are usually switched mode power supplies. These allow much more power out in a given size/weight than the conventional transformer based power supplies. They also have the potential to be very efficient, which means there is little heat generated. Switched mode supplies do have a transformer in there. But it is quite small and there are lots of other components in there. Enough on that.

The laptop type power supplies generally get warm in use. The designers know the power supply will be left on a nice woolly carpet and covered in papers. If properly designed, then they will cope with the internal temperature build up. In practice this usually limits life of certain components 1000hours or so before they degrade, but nothing more. Serious failures (and fires) are quite rare events.

A little bit of heating in the junction box will do no harm. In fact it will help keep condensation at bay.

Hope this helps.

A Power Supply with its own Dew Strap...

[jamespels]

Should be no problem with this setup but two additional points:

1. Check your RCD socket regularly.

2. Pop the IP66 box on a brick or two so that the cables run up into it. That way, any water gathering on the cables (dew, or train if you are unfortunate) will run away from the box, not towards it. Just a belt and braces safety measure but your box is only as good as the seals around the cables and this will help protect the weakest point.

[oldpink]

Not convinced I'd like to run the mains out into the garden and have it sat under a plastic tub in the snow or dew, but I admire your 'sod it, what's the worst that could happen' take on electricity, oldpink

I do a lot of outdoor gigs etc so used to using lots of electrics in damp / wet conditions

the heat from the transformers keeps any dew at bay inside the tub and the mains cable is a heavy duty outdoor cable more than capable for the job

soon I'm running a pipe out to house some network / USB / power cables to a dedicated box when I build a viewing platform / area so it will be buried under the ground

and all sealed to make it more permanent

[Paul M]

For such a semi-permanent setup I'd give a lot of thought to how the mains cable is routed.

Either buried good and deep and run through a ducting or in plain view with protection from direct sunlight.

Although the "test" button on an RCD doesn't tell you much about the operating parameters of the RCD (trip time and current) it does keep the mechanism free. This is the main reason that regular tests are carried out. It doesn't prove that it will trip in time to save a life.

Regulations changed over the last few editions of the Wiring Regulations and RCD's may now be considered as primary protection whereas they were only accepted as secondary protection previously.

I say this because ( and I'm not directing this at any one in particular!!) there is a growing belief in the general public that an RCD is everything you'll ever need or want.

My personal view is that an RCD is like PPE, a last line of defense. A good tidy installation keeping live parts wholly and adequately enclosed is the way to go. Keeping the DC voltages well and truly separated from mains is another good idea.

[Carbon Brush]

Paul M has prompted me to think about something we have not yet discussed on extension cables. Fusing.

An RCD is going to provide protection only against electricity getting 'lost'. A simple, but incomplete, description is.. What comes out of the live pin is not exactly matched by what goes back into the neutral pin - because some insulation somewhere is breaking down (damp).

However, if put put a big overload on the extension cable, and it is not faulty, the RCD will not trip. You could for example put a big heater on the end of the extension lead. The extension lead cable may well be overloaded and heat. But the RCD will not not trip.

To provide protection against overload, you rely on fuses and circuit breakers.

Generally speaking, you will have a 32A breaker in the consumer unit feeding a number of 13 amp sockets around the house. This will trip if you put lots of small loads into a lot of sockets. Or if the house wiring gets damaged and goes short circuit. For example putting a saw through an under floorboard cable. So the 32A consumer unit breaker is not much good for looking after you outside.

Your next step is the fuse in the plug at the house end of the extension lead. Extension cable safe current is determined by the cable thickness. You will see 5A, 10A, etc marked on the reels. Also the rating relates to an unwond reel so the heating from cable resistance can be easily lost. After you had a fault on a power tool, did you put the correct fuse back? Or the only fuse you could find? It is worth checking.

Putting it simply. If you have a long thin extension cable and a big fuse, then it will take quite a while to blow in the event of a fault. The large energy flow into the faulty equipment might allow something to involuntarily disassemble (explode) in the event of a fault.

So, make sure you have a fuse in the plug that is appropriate for the extension lead.

Next think about what you are going to plug into the garden box, that may have several sockets. A mains supply for a handset & motors takes little power. A 1 or 2A fuse is probably OK. A dew heater is probably OK with 2A fuse. A small hair dryer or a laptop PSU might need a 5A fuse. What I'm getting at is that you don't just accept assemblies that you pick up. Check fusing and cable rating.

The reason for rating the laptop fuse so high is the stup inrush current that is characteristic of switched mode power supplies. Your laptop might take 100watts (or about 0.4amps) from the mains when working hard and charging the battery. But when you first apply power there is a high (several amps) short pulse of current that can blow a lower rated fuse. The hair dryer has a similar inrush as the motor starts. So a larger dryer will need a larger fuse.

Hopes this is more helpful than confusing.