MOPPING UP EARTH LEAKAGE
"Typically we know earth leakage to be a bad thing. It can kill you.”

By Colin Rendell
(Published in CX MAGAZINE - JANUARY 2008)

Stray current travelling down the earth wire has often meant that someone let the work experience kid change the light in the swimming pool again. Luckily for several years now we’ve been able to rely on the protection of Residual Current Devices (RCDs) to detect stray currents and isolate circuits. But what if a little earth leakage wasn’t always a bad thing?

As technology advances, devices are getting smarter, using more and more logic circuits, motors, fans and other elements that rely on low voltage DC power. Consequently switch mode power supplies (SMPS) are frequently used to convert from 240V AC (or 110V) to a range of different DC voltages. Commonly 12V and 5V supplies are required and SMPS exist in many different devices including computers, moving lights, and splitters, and are found in LED panels. The main disadvantage of this is that due to the way most switch mode power supplies work, they leak a small amount of current to earth as part of their filtering process. This current may perhaps be at other than the 50 Hertz supply frequency.

Significantly simplified, the fast switching involved in the conversion can cause excess electro magnetic interference to anything in close proximity of the power supply. The filtering process incorporates a small series inductor in the phase and neutral lines and a capacitor between earth and each line. Consequently, each SMPS can allow a small amount of current (up to 3.5mA depending on the supply) to leak to earth. In this case, a little earth leakage actually prevents a lot of interference, so where’s the problem?

TRIPPIN’ OUT

Often the more complicated the device, depending on the design, the more SMPS’s are used. This means there’s more residual current travelling down the earth wire. Moreover, the greater number of devices being used causes residual current to propagate again. The problem for us is that if the sum of all the residual current exceeds the rating on the RCD (normally 30mA) and we trip out our supply. E.g. if a device has 5 SMPS each with an earth leakage of 3.5mA and you are using 10 of these devices you will exceed the average rating of the RCD and cause it to trip (isolate). E.g. 5 (SMPS) x 3.5mA (residual current) x 10 (devices) = 175mA.

Though an excess of residual current from multiple SMPS may be the cause of an RCD tripping, the first step when we find ourselves tripping an RCD is to ensure that the RCD is doing its job and saving lives. Check each device to see if it has any known leakage parts and ensure you know which devices are the most likely culprits.

This occurrence is becoming increasingly more common, affecting a wide range of industries and occupations. Lampies, Videos and to a small extent Noise Boys (and girls) are all affected by this problem. It is becoming common to see large racks of equipment in venues, considering how many devices are connected using a large amount of equipment that relies on low voltage. So how do we fix it?

REHAB

Most distribution devices these days require an RCD to be fitted by law, and the majority of organizations adhere to this. The best solution to this problem is to carefully calculate new loads that contain earth leakage and have them spread across separate RCDs. There is more than one flavor of protection device and there are different ways that these devices can be implemented. It is common for distros to have one RCD across the entire supply, however by using 3 RCDs, one per phase you can essentially divide the earth leakage across the phases.

Another type of protection device is the Residual Current Circuit Breaker (RCCB). This device is a combination circuit breaker and RCD in one. Though expensive, modifying distros to use an RCCB per circuit greatly removes the tripping hazard. An important thing to consider with these devices is that RCDs may be used further down stream at the supply to the distribution board (distro). Using an individual RCCB per circuit means that you don’t require an additional RCD and thus the supply is only to be used to distribute to devices that already contain protection – then it is safe to remove RCDs from supplying devices such as mains distros. This is only because the protection is being moved and not completely dissected.

These implementations have added benefits. By having multiple isolated protection points you are not putting all your eggs in one basket. E.g. if during a show an incident occurs where a single device fails and trips the protection device, if the protection is only applied to that circuit it will only isolate that circuit. In juxtaposition if the protection device is across the entire supply, a one failed device could isolate several devices where it is unnecessary. Simply put, the end concept is to trip one, not all.

MAINTAINING BALANCE

Switch Mode Power Supplies (SMPS) generate ‘harmonics’ which are multiples of the mains frequency of 50Hz. Flowing in throughout the active and neutral can be 100Hz, 150Hz, 200Hz etc in varying quantities. As stated above, the very high frequencies generate noise, which is filtered out with capacitors. The three phase power system relies on the active conductors canceling each other out. E.g. On a three phase load with each phase at 100A, because each phase is 120 degrees apart (360 degrees in a circle) then they add up to zero in the neutral. This is why the neutral conductor does not have to be three times the size of each phase. If the three phases are the same current then the neutral is zero. This highlights the need for balancing loads across the phases.

This is why for devices operating at 50 Hz, and if there is a harmonic of 3 times 50 Hz (150 Hz, 300 Hz, 450 Hz) – they can add this unwanted noise into the supply neutral. In an office building full of PCs using SMPS the third harmonic will be adding up and overrates the neutral conductors and the distribution transformers. This could result in a calamitous incident in which transformers are destroyed or fires are started.