eec247 Kevin's Story

KEVIN'S STORY

A LITTLE KNOWLEDGE IS A DANGEROUS THING

Kevin, armed with a copy of the Reader's Digest DIY manual added a socket in his kitchen over the weekend.

Kevin can turn his hand to anything and works as a handyman where he "Does Electrics" for people too.

Perception v Reality
Perception	Reality
Electricity is safe, it's not dangerous like gas.	A "GAS Explosion" gets lots of coverage on TV because there's always good visual images of a house with a side wall missing where you can see into the upstairs rooms. On the other hand, just before the jolly bit at the end of the news, we get "There was a fire in a house last night and it's believed the cause was an electrical fault ". Obviously without pictures - after all, there's nothing interesting to see. Both incidents end with the same result and everybody remembers the "GAS Explosion", but few remember the "fire caused by an electrical fault". Where gas is only present in a few places in your house (kitchen, central heating boiler, gas fire in living room), electricity is in every room, including bedrooms & bathrooms. You can't see or smell electricity and faults can go undetected until it is too late. The electrics in your house should be checked at least every 10 years according to the Institute of Electrical Engineers and a report on it's condition obtained. A typical example of such a report is on this page of our website.
Anybody can do electrics, it's not like gas - you need a professional for that.	It's true that most people can "do electrics" to a level that gets something working. Unless you have the relevant knowledge & skill, there is a strong possibility that what you have done is created a situation that has the potential to kill you or somebody else.
Even if something does go wrong, all that happens is that a fuse will blow.	Not all serious & potentially fatal faults are prevented by a fuse - follow this story to the end.
I've had loads of "belts" off the electrics over the years and I'm still here.	In cases of non-fatal electric shock, the subject is: In good health. Is partly insulated by a dry floor to earth. The current path through the body takes a route that does not flow through vital organs. The time of contact with the electricity is short.

Simplified circuit diagram of an undetected 22 ohm fault in earth path

This simplified circuit diagram illustrates an undetected 22Ω fault in the earth path of the new kitchen socket.
It could have been an existing fault, or a new one created when the new kitchen socket was fitted.
As the work was not carried out by a professional electrician, we will never know.

A plug in mains tester with three lights to find faults

Here we simulated the same undetected fault by fitting a 22Ω resistor in the earth path of a test socket.
Testing the socket with a "Plug with 3 lights" tester gives a "correct" result.
In fact, worse than that, it gives a false sense of security , but in this case we know for sure there is a fault.

The Electrical Safety Council recently published a "Best practice guide" on the "Selection and use of plug-in socket-outlet test devices. A copy of the guide is available for download from their website here.

Measuring the actual earth fault loop impedance with a professional instrument

Using a professional electrical measurement instrument detects the fault as expected.
There are no green for good, red for bad idiot lights though.
Knowing how to use the instrument and interpret the readings takes knowledge & skill.

Note: This is a simulated fault, where we know what and where the fault is.
In a real life situation, other "dead tests" would need to be performed in order to
eliminate other possible faults before performing a "live test".

Measuring the prospective earth fault current with a professional instrument

Another test to measure the prospective earth fault current shows that if there was a short
between the live and earth wires, a current of 10 Amps would flow.

The worrying thing is that a fault like this, drawing 10A will NOT blow a 13A fuse (or a 15A, or higher rated fuse).
The fuse is a safety device, intended to blow removing the danger, by disconnecting the live feed in the case of a fault.

Remember the perception: "even if something does go wrong, all that happens is that a fuse will blow" ?
In this case, the fuse will not blow, resulting in a dangerous condition.

Animated circuit of a dangerous electrical fault condition

This fault remains undetected as long as there is no connection between the live & earth wires
at the faulty socket or elsewhere in the system, where the 22Ω stray resistance forms part of the circuit.

Unfortunately, Kevin bought a toaster from a car boot sale and plugged it in.
The mains cable was damaged just as it entered the metal body of the toaster and the live wire had made contact
with the case making the metal body of the toaster live.

The 13A plug top fuse would normally blow, removing the danger but in this case and as we have shown above,
the maximum fault current is 10 Amps and the fuse does not blow.

When Kevin touches the toaster, he received an electric shock.

The shocking science behind an undetected earth fault

The shocking science behind the fault.
Using simple GCSE science (Ohms law), we can see that:
There are two paths for the fault current to take -

Path 1 (the Orange route): Live to Earth through the wiring -
The current due to the fault is 230V ÷ 22Ω = 10.45 Amps (very close to our 10A measurement).
We expect the calculated current to be higher as we have not accounted for the extra resistance of the cables etc in our simple circuit model.

Path 2 (the Red route): Live to Earth through Kevin's body -
Assuming Kevin's body resistance is 400Ω (a typical dry value) and 100Ω for the earth path resistance (another typical value),
we can see that the shock current that is delivered is 230V ÷ (400Ω + 100Ω) = 0.46 Amps.
As the full mains voltage is not across Kevin's body, there is a minor adjustment to make.
The current through Kevin's body is 0.46 Amps, with a Resistance of 400Ω, so the shock Voltage is:
0.46A x 400Ω = 186 Volts.

186 Volts at a current of 0.46 Amps is fatal if delivered for more than a fraction of a second.

The possibility of the electric shock being fatal are greatly increased if:

Kevin was standing on a wet kitchen floor and/or had wet hands.
Kevin had better contact with earth e.g. touching the stainless steel kitchen sink while touching the faulty toaster.
Kevin had one hand on the stainless steel kitchen sink and the other hand on the faulty toaster, allowing the current to flow through his heart.
Kevin could not let go of the toaster, prolonging contact with the mains voltage.

Remember the perception "I've had loads of belts off the electrics" ? This one could be Kevin's last!

The burning science behind an undetected earth fault, leading to a fusebox fire

The burning science behind the fault, leading to a fusebox fire.
Using more simple GCSE science, we can see that:
Wherever this fault is, it's using power and that level of power is:
230 Volts x 10.45 Amps = 2403 Watts (2.4kW)
(or 230 Volts x 10 Amps = 2300 Watts using the measured fault current).
So that's the same power as a standard built in conventional electric oven or a 2 bar electric heater
being consumed somewhere in the wiring of the house.

It's only a matter of time before it shows itself.

Kevin's house was built in 1976 and still has the original fusebox.
This type of fusebox was introduced in the late 1950s and variations of the same type used until the late 1980s.
There is no RCD protection (i.e. no earth leakage fault protection) in this type of fusebox.

This donor unit was complete, in good condition and with all the covers present and an ideal candidate for an experiment.
We mounted the fusebox on some treated plywood and wired it up with cables of the correct age and type (PVC)
before creating a fault inside to consume 1kW of power (less than half the power of our simulated fault created above).

The result of the fault in the fusebox led to a fire.

Note that this type of fusebox is normally found in the cupboard under the stairs
or in the garage, where rising heat & flames has the possibility of spreading the fire.
Also, once burning chunks of fusebox fall to the ground (around the 26 minute mark in our test)
anything stored below the fusebox (newspapers, carpet, tins of paint etc), gives rise to more danger.

The Electrical Safety Council recently produced a label to be fitted at the "Electrical Intake Position" saying:
"Do not store combustible materials near to this equipment
Avoid storing any items near to the equipment which are likely to cause damage or impact".
Details of the label and the "electrical fire safety campaign" are available here.

Once the fire burned out and the ash cleared away, this is what were left with.

Although this fire was created using a simulated fault under controlled conditions
there are details of real electrical fires on the East Sussex Fire & Rescue Service, Black Museum webpage
where the final outcome are remarkably similar to our test.

A real customers fusebox showing signs of starting a fire

This real life fusebox shows signs of starting a fire.
Here's how the story goes:

A local handyman changed the electric shower in the bathroom.
The original shower was rated at 6.5kW, but the new one was rated at 10.5kW.
The handyman did not upgrade the cabling or the fuse for the shower.
Shortly afterwards, when the shower was used for more that 10 minutes, the fuse would blow.
The handyman's solution was to replace the fuse with a plug in circuit breaker.
The circuit breaker tripped every time the shower was used.
When the circuit breaker tripped, it was reset and the shower used again.
Eventually the circuit breaker and fusebox could take no more, resulting in a small fire.
The handyman's solution was to blank off the fire damaged fuse holder and move the shower to the next circuit.
The new circuit breaker still tripped every time the shower was used.
The home owners then called a " proper electrician " to see why the circuit breaker kept tripping.

Closeup of a real customers fusebox showing signs of starting a fire

This shows how close to disastrous the situation had become.
We also found no RCD protection for the shower and various other faults.

WHAT THE PAPERS SAY

Here is a small selection of local stories that made it into the newspaper.

Local newspaper item about an airing cupboard fire, caused by an electrical fault

A local newspaper report of a fire caused by an electrical fault.

Local newspaper item about a fire in an electrical cupboard

Another local newspaper report of a fire caused by an electrical fault.

Local newspaper item about a fire in a sauna, caused by an electrical fault

Another local newspaper report of a fire caused by an electrical fault.

Local newspaper item about a carport fire, caused by an electrical fault, damaging two houses

Another local newspaper report of a fire caused by an electrical fault.

A LITTLE KNOWLEDGE IS A DANGEROUS THING

So, rather than doing your own electrics, with or without help from the various internet DIY forums where
"in the land of the blind, the one eyed man is king" or employing a handyman,
have your electrical work carried out and properly tested by a qualified electrician.