Eclectic safety definition.
Electrical hazard significant to workers, particularly when mishandled or not maintained can cause burns, shocks, and electrocution (death).Assume that all overhead wires are energized at lethal voltages. Never assume that a wire is safe to touch even if it is down or appears to be insulated. To inform a local electric utility company and emergency services.
Do consumers really understand test instruments?
As an experiment, About a ‘Microwave Leakage Detector’ that sold for 10 compared to others that were selling for around 50. Not surprisingly the 10 seemed to be selling well and I was interested to see if customers were driven by price only. Below are the answers from customers and my views on this.
The question I posed was ‘ What is the fail threshold? Does this drift with time?’ The answers I got were as follows.
Cannot really answer the question.
I bought this item to test my daughter’s microwave after popcorn burnt in it.
The heat did not cause much visible damage but I was concerned about the door to surround distorting.
On using this tester, I found the seal was indeed leaking and so condemned the microwave.
I have subsequently tested other machines and they all ‘passed.’
As for the technical details, I have no idea.
I was just happy to find the leaking microwaves and so avert possible damage to her health.
For the price, it did the job. If you required a definitive tester I would suspect the cost will be high.’
Perhaps it depends upon what you want the detector for. If you want professional quality then you need to spend quite a lot of money. If, however, you just want a gadget that will allow you to occasionally check a family microwave then I would say that this one is fine. I can’t think of any way that the fail threshold could drift because the device is simply a coil in which stray microwaves generate a small electrical signal. However, all it will do is tell you whether there is any leakage and whether that exceeds safe limits. A professional model would yield much more information that the average layman wouldn’t need or, possibly, even understand! Answer #3You need the expertise of a specialist, I am not one Answer #4You would need a microwave expert to answer these two questions. Answer #5Truthfullly, unless the door is damaged, there shouldn’t be a problem. I don’t think anyone can answer questions like this except the manufacturer. Under normal conditions, as long as you are a foot away from the microwave when it is on, there shouldn’t be any issues. This item will only measure leakage when pressed up against the microwave. Comments Any test instrument that says PASS or fails must, first of all, make a measurement and then decide whether this was OK or NOT. In order to take this decision, it has to have an internal set threshold. If it measures signals above this then this is a FAIL and it measures signals below this it is a PASS. Customers need to be suspicious of a tester that does not tell you what this threshold is. For example, if this threshold is set high, then it is going to PASS faulty appliances. If it is set too low then it is going to FAIL perfectly good appliances. This has nothing to do with price – it’s to do with whether it’s fit for purpose or not. Anyone remember the ‘Fake Bomb Detector’ story? That shows that you can indeed fool a lot of people most of the time. I also found the comment ‘I can’t think of any way that the fail threshold could drift because the device is simply a coil in which stray microwaves generate a small electrical signal ‘ quite worrying. All it would take is mechanical damage to the coil in which case it will end up PASSING all appliances, even faulty ones. My earlier blog titled ‘When is a PAT tester, not a PAT tester’ mentioned a tester that had a PASS limit 100 times (10 ohms instead of 0.1 ohms) the recommended one which would end up PASSING faulty appliances.
Most customers are trusting and will assume that testers offered for sale are fit for purpose. It is up to the professionals in the industry to make sure that we do not abuse this trust.
Avoiding serious injury from an electric shock
We get an electric shock when current passes through our body due to a voltage difference. For example, if we touch a live wire at 230V, this voltage pressure will try and push current through our body to the ground that we are standing on. This is because the mains supply in a building is always at 230V with respect to the earth.
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Effects of shock
The effect of the electric shock is dependent on the amount of current flowing through our body and also whether it is DC or AC voltage that is causing the shock. The table below gives more information on this
Current (mA) Effect
1 – 3 A slight tingling sensation
5 – 10 Painful.
10 – 16 Arm and hand muscles close involuntarily. Cannot let go.
20 – 25 Cannot breathe. Paralysis of chest muscles.
50 + Heart fibrillation: Rapid, irregular contractions of the heart muscles. Could be fatal.
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Residual Current Devices (RCDs
From the above figures, one can see that if the current can be kept low a fatal accident can be avoided. This is where RCDs come in useful. RCDs monitor the current flowing through the Live and Neutral wires of an appliance. If there is an imbalance in this, say due to the current flowing through a person to Earth, then they will cut off the mains supply to the appliance. They work at very low currents, and act very fast, typically in fractions of a second to protect the person from electrocution.
To protect a person from the risk of a fatal electric shock, RCDs need to operate at a current of less than 30mA.
The amount of current that flows through a person’s body is dependent on the voltage of the source and the contact resistance. The table below shows the amount of current that can flow in different situations.
Voltage Source Type of contact Typical resistance Current (mA) Effect
230V AC Glancing contact with live part 500,000 ohms 0.46 A slight tingling sensation
230V AC Grab live part with wet hands 10,000 ohms 23 Cannot breathe. Paralysis of chest muscles.
Cannot breathe. Paralysis of chest muscles.
From this one can see the effect water has in reducing contact resistance. This is the reason that electrical sockets and switches are kept well away from areas close to water. This reduces the chance of a serious injury in case of a fault developing.
110V transformers are recommended when working with portable tools or on construction sites. They reduce the risk of a harmful electric shock significantly. They are center tapped with the tap connected
Voltage Source Type of contact Typical resistance Current (mA) Effect
110V transformer Glancing contact with live part 500,000 ohms 0.11 Hardly noticeable.
110V transformer Grab live part with wet hands 10,000 ohms 5.5 Painful
to earth. This makes the Live voltage only 55V with respect to earth. The table below shows the amount of current flowing when a 110V transformer is in use and there is contact with Live parts.
When working in environments which is damp and the chances of damage to appliances are high (outdoors, construction sites) then use of an RCD or a 110V transformer is essential to protect the user from serious injury from an electric shock.
Round 3 pin plugs with no fuses
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These plugs (officially BS546) comes in 2, 5 and 15 Amp versions and are still used in stage lighting installations, large country houses, and hotels. They have their own dedicated circuits which are fused.
Normal 13 Amp sockets are wired in a Ring Main. Circuits containing BS546 sockets are wired “radially” back to the fuse board and are usually fused at 5 Amp or 15 Amp.
How are these to be PAT Tested?
The Portable Appliance Test is just the same as for other appliances with the 13 Amp (BS1363) fused mains plug. Identify the appliance as Class 1 or 2 and carry out the appropriate tests using a suitable adaptor.
One will not be able to check the fuse, as the plug does not have one. However, it is good practice to ensure that the wiring has been inspected recently (in the last 5 years) by a competent electrician.
The safety of an appliance with a round pin (BS546) plug is provided exactly as other appliances. The main difference is the absence of the fuse in the plug