RCDs 101: When Safety Meets Simplicity

 

The inner workings of an RCD may not be complex or fascinating but they are what makes today's electrical installations far more reliable and safer than those used in the previous century. Residual current devices as they're called are able to detect leakage current in a panelboard no matter its size. 

This way, in case any surge of current is about to come through, the RCD is what shuts down the board so that the current doesn't pass through and damage circuit breakers and outlets or potentially devices and appliances. But as with everything, there is not just one type of RCD you could have in a panelboard.

Types of RCDs

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Socket-Outlet

RCDs are not only found in panelboards, they can also be part of a socket which is why you have socket-outlet RCDs. These are a more local solution since they only secure the safety of whatever device or number of devices you have connected to the outlet a socket-outlet residual circuit device is found on.

Fixed

A fixed RCD is one that attaches to a DIN rail system which is found in a panelboard or any electrical installation. These are the most common types of residual current devices and the oldest too as RCDs were first only found added to electrical installations and nowhere else.

Portable

The above changed in recent decades as electrical components advanced to a whole new level of efficiency and portability hence the portable RCD was born. They are similar to what a socket-outlet RCD does but they are not tied to one outlet. Although that happens but only for the time being as you plug in portable RCDs into a socket which makes them applicable for temporary use only.

What to Look For in an RCD

Sensitivity

Let's start off with RCDs that have high sensitivity levels. They are often found to be the safest as they react at the slightest change in current anywhere from 10mA to 30mA. This is when the RCD trips and keeps the board and devices safe from electrocution. RCDs with medium sensitivity can register differences in current between 100mA and 300mA which makes them usable in certain industrial applications but not in households and similar residential buildings.

Medium-sensitivity RCDs are also found in retail stores and offices. If you go for an RCD with a low sensitivity then this means you have a device that trips if an increase of 500mA an upwards happens. Low-sensitivity residual current devices are meant to be used strictly in industrial applications since there's machinery that operates with a much higher power consumption from that of household appliances.

Current Rating

The current rating of an RCD, as you already know, is measured in mA (milliampers) and it tells you the maximum current the RCD device is capable of handling. This rating is important for sizing the RCD to the panelboard you want to fit it to. It's crucial that you go for an RCD with a higher current rating than the short-circuit current of the circuit in the panelboard. Since this is the maximum current the device will let through before it trips, you should have an RCD with around 30 mA more than the current of the circuit. 

Voltage Rating

The majority of RCDs are rated for 240V AC circuits but if your panelboard has a different circuit you can also get an RCD rated for it. For instance, you can go for one that is rated for both 110V and 415V three-phase units. This is important for compatibility reasons since a 240V-rated RCD won't work with a 415V circuit but it can work with a 110V one.

Frequency Rating

The frequency range in which a residual circuit device operates is quite important as it also should match the circuit in the panelboard. This is especially important if you have equipment that relies on VSDs or variable-speed drives. This is because the current harmonics being produced by these types of devices can easily make the RCD trip for no reason. That is if you don't go for an RCD that works in the same frequency range which is usually between 50Hz and 60Hz.

Categories of RCDs

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AC

An AC RCD is a type of RCD that works only with sinusoidal currents which are found in circuits that produce very little disturbance. This includes devices such as immersion heaters and lighting circuits amongst many others.

A

An A-type RCD is made so that it works in both AC and DC pulsating circuits. This makes them more versatile than AC-type RCDs. 

F

With an F-type RCD device, you can protect single-phase installations quite well. This means you can keep circuits with VSDs like air cons, heat pumps, washing machines and such safe from false tripping.

B

A B-type RCD, on the other hand, is good for three-phase installations. They are quite similar to F-type RCDs in that they can be used for about the same types of devices.

S

Using a S-type RCD means you get a certain time delay. This type of feature is what can be useful for inrush currents coming from an electric motor or a transformer circuit.