Keep the Fireworks Out of the Fusebox

Fuses are one of the simpler devices that we work with, yet techs do not really know some important fuse specifications. What you don’t know could hurt you if you use the wrong type of fuse as a replacement. Fuses have five important ratings: voltage, amps, interrupting, one-time or time delay, and finally whether or not it is current limiting.  Most techs are familiar with the volt rating and the amp rating, but a lot are unfamiliar with the interrupting rating.

Although all fuses are designed to open when the current exceeds their amp rating, this does not happen instantly. In the case of a dead short, the fuse will be subjected to a much higher level of current for a fraction of a second. For a fraction of a second, the fuse can be exposed to 100,000 amps, causing it to explode like a bomb! The amount of current the fuse can withstand and not explode like a bomb is called the interrupting rating, listed as IR on the fuse body. Inexpensive fuses have an interrupting rating of 10,000 amps – which is comparable to most breakers. Better quality fuses have much higher interrupting ratings – such as 200,000 amps.

Two fuses can have the same volt and amp rating but have vastly different interrupting ratings. If you replace a fuse which has a 200,000 amp interrupting rating with one which has only a 10,000 amp interrupting rating, you are creating a bomb. If you take look at the two fuses pictured here, you can see that they are both 600 volt, 30 amp fuses and are physically interchangeable. However, the one on the top has a 200,000 interrupting rating but the interrupting rating of the fuse on the bottom is only 50,000. They are not functionally interchangeable.

Further, the fuse on the top is designed to be a current limiting fuse. This means that it limits the amount of current that can pass downstream of the fuse. I can hear you saying “all fuses are current limiting.” Not really. During that fraction of a second after a direct short, thousands of amps pass downstream through the fuse. A current limiting fuse limits this spike. Typically, the spike is limited to 10,000 amps. The importance of this is that it can reduce or even prevent an arc flash from happening in the equipment down-stream of the fuse. This is why industrial and commercial services usually protect their service panels with current limiting fuses which have a high interrupting rating. Keep the fireworks out of the fusebox. Always replace fuses with ones that meet ALL the fuse specifications, not just the volt and amp ratings.

How Does an Air Conditioner Cabinet Become Energized?

The last two weeks I have been talking about electrical safety. Specifically, discussing the electrical hazard of an energized system cabinet. This week I would like to discuss ways that the cabinet of a system can become energized. First, for a cabinet to have a voltage other than ground, the cabinet is either not grounded, or the ground has failed. Failure to properly ground a metal cabinet is the first condition that sets up the electrical hazard. But another mistake or failed component is required to actually energize the cabinet. Some are obvious – such as a loose energized wire touching the cabinet. Others are less obvious, such as a grounded electric motor. If a motor is grounded (not shorted or open) and the equipment cabinet in which the motor is mounted is not grounded, when the motor is energized the cabinet will also become energized. Motors can sometimes be grounded and still operate. So you touch an operating unit and get shocked. Another failure that can energize a cabinet is a broken or missing insulator on heat strips. Most heat strips have one side that is wired hot all the time. If the ceramic insulators break and allow the strips to contact the frame holding them, the cabinet can become energized.

A sneaky way for a cabinet to be energized is to wire a 110 volt device into a 230 volt unit using one leg and ground. Basically, current will be going through ground anytime the 110 volt device operates. This alone won’t cause an energized case. But if the ground between the unit and the panel breaks or just gets a bad connection, now there will be a voltage between the case and the actual ground – and the case will be energized. To avoid this, don’t wire 110 volt devices this way. You either need a separate neutral AND a cabinet ground (4 wires), or you need a 230 – 110 transformer. This is why newer electric dryers have 4 prong plugs: two for the 230 volt hot legs, one for neutral for the motor, and one for a cabinet ground.

So here are a few simple rules to help you avoid creating an electrical hazard:

1. All metal cabinets and pipes should be grounded
2. Ground wires should never be used as part of an operating circuit.
3. When equipment has both 230 volt and 110 volt loads, the equipment either requires a neutral wire AND a separate ground wire, or a 230 volt to 110 volt transformer.