Multiple extension leads on plug sockets


Well-known Member
Thats a good example to give. However, the length of the cable or its resistance is important, but the temperature it is being used at, is equally important. As these two factors alter the volt drop at the end of the cable.

As a water heater is a fixed resistance only, it will still operate at a reduced voltage (i.e. < 5% x 230V). If the voltage is reduced, then so is the current. As the temperature of a cable increases when a load is applied, then its resistance increases, which further affects the volt drop, which further effects the current. I've tried to do some calculations, using a 50m cable reel of 1.5mm flex. Two x 2300w water heaters attached to said reel, would still draw enough current to blow a 13A fuse. But I have no reason to doubt your recollections.

Temperature under loads are an important factor in cable designs, such as cable reels. Manufactures will stipulate the maximum loads for their cable reels, typically 3000w unwound & 1500w coiled (50m). The coiled load is limited as the cable is unable to dissipate the heat generated enough to operate safely, as demonstrated by your picture. Perhaps this cable reel, did not have the thermal safety device fitted.


Well-known Member
One issue with using extension leads, especially if you cascade to other mains extension leads, is the issue of earth leakage current. Every device has earth leakage current or touch leakage for protection class II devices. A typical protection class I device (earthed) will have a leakage current of 0.8 mA. If the earth for the appliance breaks and someone touches the appliance the 0.8 mA will flow through them, without hazard.

The maximum permitted earth leakage current is 5 mA (3.5 mA as far as most products are concerned). This is to limit the potential for electric shock if the earth breaks, which is possible and will often go undetected.

If you plug 10 items into a mains extension, and each has 0.8 mA earth leakage current, the current flowing in the extension cable earth is 8 mA. Should the extension earth break then anyone touching ANY of the devices connected to the extension will have 8 mA flowing through them, which may be hazardous!

This is often something that is overlooked when using mains extensions.

NOTE: Class II (double insulated) will typically have the touch leakage limited to 0.01 mA.


Distinguished Member
I guess the only good news is that even if the earth fails, the RCD will still trip at 30mA - which is consider to be below a lethal current. RCDs do not have an earth connection and work simply on the difference between the current flowing in the neutral and live connection.

Most of the time, leakage current tends to be high impedance parasitic "tingles" that are of no harm. Higher currents tend to be due to a fault or breakdown in insulation. I have an old jewellery polishing machine that tripped out the RCD. Turned out the insulation on the stator windings had started to break down and this was the cause of the fault.


Well-known Member
Depending on contact surface area, body resistance, and age a current above 5 mA may cause muscular contractions. Certainly at 10 mA it may cause your diaphragm to become paralysed, eventually leading to suffocation! The 30 mA of RCDs is to prevent death by stopping the heart (causing ventricular fibrillation, VF).


Well-known Member
@xmb makes a good point that an accumulative affect of intentional earth leakage and open circuit earth on appliance, can give a recipient an unpleasant experience, without a 30mA RCD disconnecting the supply.

Not sure whether the use of multiple leads would worsen this?

Edit; see what xmb is suggesting now.
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