Tag: safety

Mains safety: How not to pop the breakers.

Posted by on December 8, 2011

So you’ve been troubleshooting some device or fooling around with a mains circuit and pop, goes the breaker. Well OK, let’s discuss a few ways of dealing with this situation.

Here are some common methods:

  1. Fuses
  2. Ballasts
  3. Secondary, lower rated breaker
  4. Dedicated line


The methods explained:


The first and probably simplest method is to use fuses, however it soon becomes apparent that it’s also an expensive idea. Fuses should be, in my opinion, one of the last defences.

Another problem comes from an improperly wired plug (or non polarized as is the case in some countries) then you could have an open fuse by the netural side, but your device could still be at live potential! — This could be lethal.

So even if you’ve got an open fuse, don’t assume it’s safe to get your paws in there just yet, unplug it first. Likewise with power switches, if they aren’t bipolar then don’t assume, be smart.


As simple to implement as a fuse, but it won’t cut-out if you short the line, it will just provide whatever current the ballast lets through.

A ballast can be inductive or resistive, typically you’d want to use a resistive ballast, examples are room heaters, incandescent light bulbs, etc.

The down-side here is that if you were to use a ballast big or small enough, you can actually cause more damage with it than without it.


A “big” ballast (low value, allows for a lot of current) will most likely damage your device in case of a dead short, because it will continue to use your device as the current path, whereas a breaker would most likely trip open very quickly. Now if your device failed short, something already went wrong, but if you keep powering this device, chain reactions can occur and more damage can be caused. Because of this, sizing the ballasts properly is very important.

A “small” ballast (high value, allows for very little current) will display a noticeable resistive divider behavior between itself and the load, this means that the load (device) will see a lower voltage and this can cause trouble in certain cases, for example poorly (or cheaply) designed switch mode power supplies can have it’s switching / rectifier sections fail due to over-current (lower voltage forces the controller to use a higher duty cycle to compensate for this, thus longer “ON” times are seen). This can actually lead to yet another chain-reaction event.

Remember, as the smps starts up it needs to charge the input filter capacitors and this is a relatively heavy load, at this stage the voltage on the power supply may be way below it’s minimum specified rating thanks to your ballast.

Secondary breaker

You can wire up a box with an inlet and an outlet with a small breaker in between, typically breakers have a higher trip current than rated to allow for high inrush currents as seen on inductive loads such as motors, etc. So definetly do take this into consideration.

One issue however is that lower rated breakers can either be harder to find or more expensive, so keep that in mind as well.

Dedicated line

Having a dedicated line just for testing devices is ideal, if you add a secondary breaker and a properly sized ballast where applicable, you’ll never have to worry about blacking out or damaging something ever again.

You can add your own ground fault (switchable in and out) as well as residual current devices, etc. I would even suggest implementing phase indicators like you’d find on most properly installed residential panels.


It should be clear that a combination of all methods can only be ideal if it suits your needs, so there is no definitive answer here. Investigate, experiment and figure out what’s best for you.



DIY – Acrylic Cement

Posted by on July 27, 2011


As part of an upcoming series of posts on “diy project enclosures” I will describe how to make acrylic / Perspex cement at home with very few elements. This very same technique may be applied to other similar plastics as well, although in such cases the outcome is almost always a “putty” instead of a proper adhesive.

To make your own acrylic cement you’ll need the following materials:

  • Acrylic bits / scraps (These will be sacrificed in our solvent)
  • Glass jar with a lid (Must be perfectly clean and the lid must seal properly!)
  • Acetone (Low purity, 99% is more than fine)
  • Humongous cup of coffee (Thou shall drink the coffee!)

How to obtain the cement:

The process is quite simple, all we have to do is place the scraps into the jar, pour some acetone until the acrylic is fully covered and simply close the lid. Stir it every few hours by wiggling the jar, about a day or two after you’ll have a powerful cement for acrylic. You may drink the coffee now!

Notes and observations:

The acrylic bits have to be small, the bigger they are the longer it’s going to take for them to dissolve into the acetone. I reckon if you were to grind the acrylic into a dust, it would work almost instantly — However finding a safe way of doing it may be challenging (Old blender, coffee grinder perhaps?) — The dust may not be very healthy though.

A few words on safety:

Keep the acetone and glue away from children and pets; it is quite flammable and thus inherently dangerous if mishandled. Always store the containers fully closed and away from ignition sources. It’s worth noting that casual exposure to acetone is “a-ok” and it will merely dry your skin.

That’s all there is to it, enjoy and be safe!