Using Power Leads: Facts & Regulations

Early caravans used appliances, particularly refrigerators and air conditioners that drew more energy than today’s equivalents. Australia's caravan parks thus used 15 amp outlets. Few RVs draw as much as that now, but 15 amp circuits are still obligatory. Because of this most camper trailers, caravans and motor homes have 15 amp plugs, cabling and socket inlets, plus residual current (safety) devices and 15 amp circuit breakers. This ensures there is no weak link. Having a 15 amp appliance draw that much current through cables and connectors of only 10 amp rating is like having a 15 tonne snatch strap and a 10 tonne shackle. (The RCD and circuit breaker, together with a master switch is often combined in one unit, known as an RCBO).

Some RVs have only a 10 amp supply cable, RCD (Residual Current Detection) and circuit breaker/s. This works fine because a 10 amp plug will fit a 15 amp socket outlet – but a 15 amp plug will not fit into a 10 amp socket outlet. It is also electrically safe as the RV’s obligatory 10 amp circuit breaker limits current through the 10 amp cable accordingly.

Joining electrical supply cables together to extend length has always been dangerous. It has also been illegal for some years. The leads must be of one unbroken length. If your cable is not long enough, you must either obtain a longer one, or forgo the use of site-supplied power. None of the above is negotiable. These are obligatory enforceable requirements so personal opinion and interpretation is not relevant.

You are legally obliged to have a safe complying cable (as defined in AS/NZS 3001:2008 – as amended in 2012) and to use that, and only that, to connect your vehicle directly to the socket outlet. If someone has inserted a double adaptor, try to resolve this amicably. If that fails, have the manager insist it be removed.

The allowable supply cable lengths were revised in 2008. They are shown as Table 5.1 in the above Standard – and outlined here (click on the image at the beginning of the article to enlarge). As will be seen, if one can cope with a 10 amp supply, it is possible to have a cable 100 metres long – but as it must have 4.0 mm2 conductor it is heavy, and bulky to store. If used the whole length must be uncoiled – or the remaining coiled cable may overheat. (That allowed is derated (by approximately 40%) if used for loads that have very high starting current – such as an air compressor starting up on full load. Such loads are not likely to be encountered in RV use.)

Clause 64 of the Occupational Health and Safety Regulations requires that, in a place of work where an item of electrical equipment is, in its normal use, subject to operating conditions that are likely to result in damage to the item, that it be tested and tagged by a ‘competent person’ to ensure it is safe for use. The testing procedure is dictated by AS/NZS 3760. Tagging is not required (under legislation as of May 2013) for private use – e.g. in one’s home, but caravan parks etc now insist it be done.

Many RV owners need to use power where only a 10 amp supply outlet is available and made up illegal 'cheater cables' (i.e. that have a 10 amp plug and a 15 amp socket). This defeats the intent of the 15 amp plug’s larger earth pin: to make such connection impossible. Doing this is illegal and potentially dangerous. Why it can be dangerous is a little complex – but is explained below.

The so-called Amp-fibian unit, a cable that limits current drawn from a 15 amp supply to 10 amps, may however legally be used to resolve the situation. The latest AS/NZS 3001:2008 appears however to preclude its use to extend supply cable length of the permanently attached 10 metre, 10 amp supply leads fitted to some caravans and motor homes. The Victorian safety authority advises (in writing) that this is legal – but advice from another State (also in writing) is that it is not! The AS/NZS 3001:2008 Standard’s 2012 amendments address this very issue, both in text and graphically, and appear to confirm the general consensus. (If/when this matter is resolved, the article will be updated accordingly).

It is legal to use a 10 amp supply cable if the RV’s inlet socket, RCDs, and CBs are changed to 10 amps – thus preventing over 10 amps being drawn. No major Australian supplier has a direct 10 amp socket inlet replacement but Clipsal's 56 A1310 surface mounting 10 amp inlet socket is (in Australia and New Zealand) a legally acceptable alternative – but not a cheap one.

It is also legal (subject to waterproof storage, physical cable restraint etc) to have the cable permanently fixed at the RV end. This cable must be either connected to the line terminals of the circuit breakers, or to some other permanent facility such as an approved junction box. The above is covered by section 3.2.3 of AS/NZS 3001:2008. Cable lengths etc remain as in Table 5.1.

Earthing was until recently, the primary safeguard against electrocution. It still plays a role but, in most issues, so-called RCD (Residual Current Detection) is now primarily effective in saving life. The units constantly monitor and compare the amount of current flowing in the active and neutral cables. Unless they are totally equal there must be some flow to earth. If that is detected, current is cut off. The above can happen, say, if Uncle Jack changes a dead light globe without realising the light switch is still on - breaks the glass and contacts a live part. Or accidently sticks his finger into the live socket. Current then flows to earth – via Uncle Jack. If polarity has been reversed (i.e., active and neutral reversed in the wiring to that switch, that globe will still be live even if the power switch is turned off). As that risk is high where people (illegally make up their own supply cables) all Australian RVs must have power switches that have so-called ‘double-pole switching’ – i.e. both active and neutral are switched.

Prior to RCDs, the probability of such electrocution was high. But an RCD detects current flows in one lead, but not the other – and cuts the current in time to avert death.

The circuit breaker’s main function is to ensure that excess current is not drawn in the event of the active and neutral leads accidentally touching, an appliance failing such that it draws excess power, or simply the user attempting to run appliances drawing more than (in this case), 10 or 15 amps. If the supply cable’s resistance (or more correctly impedance) to the flow of current is increased (by co-joining leads etc) the circuit breaker will be slowed, or not even operate at all.

This is why supply cable specifications are now rigidly enforced. All of the cables allowed enable the system to operate in time. Longer cables are also thicker (so all, regardless of length) allow the protection to act as intended. That is why cable length must not be extended, nor cables joined together.

For RCDs to protect, the supply cable’s resistance (or more correctly impedance) to the flow of current must be within specific limits to ensure the RCD acts within 0.4 second. If that time is exceeded, the RCD may still operate - but often too late to save life.

This is why supply cable specifications are now rigidly enforced. All of the cables allowed enable the RCD to cut off the current in time. Longer cables are also thicker (so all, regardless of length) allow the RCD to act as intended. That is why cable length must not be extended, nor cables joined together.

Earthing is still required. It may carry initial fault current before the RCD and/or circuit breaker cuts off supply, but it plays no direct role in the RCD's action. It is also only partially effective against protecting against contacting live metal because, unless done 100% effectively, some part of the fault current may flow through to earth via anyone contacting such metal (i.e. by forming a parallel path).

The changed AS/NZS supply cable rules are consequent to the major reliance on RCD (Residual Current Device) and Circuit Breaker protection as well as earthing.

(The above is sourced from Standards Australia's documents).

The author has a practical and theoretical background in designing and building large electrical systems but is not a qualified electrical engineer, nor licensed electrician). As will be seen, there is more to camper trailer, caravan and motor home electrics than may at first seem. It is wise to keep yourself as informed about your vehicle as you can to know what to do in the event of a breakdown or failure but never perform any rewiring yourself as this must be done by a licensed auto-electrician.

Collyn Rivers is an engineer/writer/publisher. His major books in the RV area are the recently totally rewritten, Solar That Really Works, The Camper Trailer Book, and the all new Caravan & Motorhome Electrics (the successor to Motorhome Electrics); and the Campervan & Motorhome Book. These books are written in everyday English, yet recognised globally as technically impeccable. They cover every aspect of their subject matter in detail. All are available from ExplorOz.com