What are safety switches and why use them.
Safety switches are properly known as:
RCD – Residual Current Devices.
RCBO – Residual current Circuit Breaker with Overload protection
They can also be called Earth Leakage Device, or Earth Leakage Circuit Breaker (ELCB).
How do they work?
From an electricity perspective, what goes in, must come out.
Power going in to an appliance enters through what we call an active (A) conductor. It returns from the appliance via a neutral conductor (N). The power going in to an appliance is equal to and opposite to the power leaving the appliance. If an imbalance is detected, the RCD or RCBO will detect this change and switch itself off.
RCD/RCBOs are designed to switch off (trip) within 300ms of a fault being detected. This is equivalent to 15 electrical cycles and slightly less than one heart beat (for most people).
Why do imbalances occur?
Generally an imbalance occurs because there is what we call leakage to earth.
Leakage to earth may occur when, for example:
- a child sticks a paper clip into a power point
- a person touches a live cable, (e.g. sticks their finger into a light socket by accident)
- a person drops a hair dryer (or similar) into a sink, tub, bath, toilet etc.
- an adult hanging a picture frame up, hammers a nail through a cable
- a motor is starting to break down in an appliance and electricity is entering the chassis (e.g. fridge)
- an element within an appliance is breaking down and leaking electricity to the chassis (e.g. kettle)
- a pool pump starts to break down leaking electricity into the pool water
- a cable starts to break down and leaks electricity into an appliance chassis
- and many more examples
The first four points above are examples of where we might be most reliant on RCDs for safety.
If working correctly the RCD will trip which increases the likelihood that your life or the life of your loved one will be saved.
Without a safety switch, the chances that you will receive a lethal electric shock increases dramatically – particularly for children!
The other items that are listed following the first four, are instances where appliances are getting old, need maintenance or are becoming a potential safety hazard. Your RCD is there to protect you in these instances as well.
The Australian Standards specify that all new circuits are to be installed with RCDs as the circuit protection. These specifications also apply to alterations and additions.
While there are some exceptions, the general rule is that you install an RCD onto any new or altered circuit for the protection of occupants.
New rules that are soon to be released, are even more onerous. These essentially specify that all new work or alterations will require an RCD to be installed – with fewer exceptions.
This adds some cost, but increases household safety dramatically.
Do they have to be tested regularly?
The short answer is Yes!
Isn’t that costly?
There are two ways to test if your RCD is working correctly. One is 100% reliable, the other is less accurate.
The only certain way to determine if an RCD is working correctly is by using specialised testing equipment. This requires trained personnel, with expensive test equipment.
This equipment will send a designated leakage current through the earth to the RCD which should be detected as a fault and cause the RCD to trip. The instrument should then calculate the trip time and record it.
For complete accuracy (and your safety), the equipment should be capable of testing in the 0° and the 180° voltage phase.
Ask your trades person if their instrument will test both positive and negative voltage phases. (If they don’t know what you are talking about, choose another trades person).
The less reliable method is to use the test button on the RCD at your switchboard. This is less accurate as it is an un-calibrated leakage current, it cannot calculate the trip time, and it cannot specify voltage phase for testing.
Your RCD may trip out slower than it should; trip at a higher current than is safe; or not operate correctly in both phases.
The test button will NOT determine this
Test equipment is the only way to determine if your RCD is operating safely.
While this is less reliable (and hence less safe), it is a satisfactory and cheap method for home owners to undertake themselves on a monthly basis.
And I encourage them to do so!
However while this may be satisfactory on a month by month basis, it is highly recommend that formal testing be undertaken by trained personnel with specialised instruments. This will certify that the RCD is operating correctly, and will protect the lives of yourself and of your loved ones.
While our wiring rules specify 300ms as an upper limit for trip times on RCDs, should any test that I undertake be higher than 100ms, I would be discussing this with my client and encouraging more regular monitoring, or replacement – particularly if small children are on the premises.
As mentioned above, 300ms is 15 full voltage cycles. 100ms is 5 full voltage cycles.
The longer electrical current is passing through your body, the more likely damage will occur. Also, the longer that current passes through your body, the lower the resistance to it becomes. This results in faster and increasing damage to your body with time.
Also, the younger the child, the lower the resistance to electricity and the more likely damage will occur. Therefore the shorter the trip time the better – in my opinion.
Keep this in mind should you be advised to change your RCD 🙂
Bye for now,
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