Download Portable Generators- to earth or not to earth

Portable Generators and Inverters- to earth or not to earth!
Background
While electrical safety in general has improved due to the widespread use of the RCD (Safety Switch), portable
generator and inverters and the provision of electrical protection therein, continues to be a topic of great debate.
Mobile generators and inverters are now used more frequently as alternate power supply for a variety of reasons
such as convenience, cost savings or as a revenue raising option to grid feed supplies.
There are many applications for portable generators and inverters including Mining, Construction, Disaster
Control, Emergency Services, Agriculture, Vehicular and of course the Recreational and Domestic Industries.
Users of this equipment confront many challenges with regard to effective personnel and equipment protection
and often confused messages with regard to regulated and conventional practices.
The usual method of ‘earthing’ for electrical protection, through the use of an earth-stake, has in itself generated
great debate over whether to ‘earth or not to earth’ a generator. There is a consensus, from some in the industry
that “The only use an earth-stake will achieve is to stop the generator from rolling down the hill”. It should also
be noted that it is not always practical or realistic to put down an earth-stake when it comes to using a mobile
generator or inverter. Consider the following scenarios where:

No earthing mechanisms are available. (A hired generator),

Environmental issues prevent adequate earthing – (hard rock, sand, rubble, concrete and asphalt.)
RCD’s require a good or low impedance earth path in order to operate,

The testing of the earth network is “electrical work” under all jurisdictional legislation, unfortunately
licensed workers are not always available to legally undertake this work,

Time and cost constraints do not allow for the construction, measurement and testing of an adequate
earth,

Risk of damaging other underground utilities.
Australian Standards
Generator standards in Australia (AS/NZ 3010: 2005) stipulates two techniques for safe operation, the first is
the use of neutral bonded generator using an RCD isolating in 30mA for electrical protection. This technique
requires the generator to be safely earthed for guaranteed performance and this is not practical or possible in
many situations as previously mentioned.
Secondly is the use of an isolated secondary winding using the generator to create electrical separation is
preferred in some industries (e.g. mining) but this means only one class I appliance at a time can be safely
operated from the generator. In some industries the use of class 2 appliances on isolated generators is not
allowed.
The Australian Construction Standard AS/NZ 3012: 2003 mandates that only generators fitted with an RCD
rated at 30mA (neutral bonded ) are allowed on those sites, which then leads to the question what defines a
‘construction site’? Does SES or CFA personnel fighting a burning house represent a construction site? It could
also be argued that it is a demolition site.
Generator & Inverter Configurations
Generators and inverters are supplied from the manufacturer, as either a bonded neutral to frame type or an
isolated secondary winding type. From a lay man’s perspective, it is hard to distinguish between these two types
of generators. The confusion does not just stop there, the use of standard RCD power boards have been banned
in some industry sectors, as they create a possible dangerous situation with isolated generators leading to
individuals being falsely protected with the common perception that RCD’s will provide protection in all
conditions, where in reality, this is certainly not the case .
Isolated generators, in essence represent an IT (unearthed/floating) earthing system, where all active parts are
either insulated from earth or one point is connected to earth through a sufficiently high impedance. The
exposed conductive parts of the electrical installation are either earthed individually or collectively. But with no
neutral/earth bonding the RCD will not function on an isolated generator with or out an earth-stake installed.
AS/NZ 3000:2007 7.4.7.4 mandates that an isolated supply shall have no connection to earth. Other
discrepancies between Australian Standards, further inflame the confusion and undermine the knowledge base.
As an example, AS/NZS 3001:2008 (Transportable Structures) stipulates for portable Generators or Inverters:
2.4.1
Portable Generators or Inverters shall be one of the following types;
(a) Isolated type
or;
(b) RCD Protected type
Note the discrepancies with AS/NZS 3010:2005 in Figure 2.1 (above). Further discrepancy lies between
AS/NZS 3001:2008 and AS/NZS 3004.2:2008 (Recreational Boats), which stipulates;
6.4 IT type systems are permissible for a single fault between a live part and exposed conductive part to occur
without automatic disconnection, providing a continuously operating earth fault or insulating monitoring
system, with automatic disconnection is provided;
Note the discrepancies with AS/NZS 3001:2008 in Figure 2.1 (above).
IT earthing system
The IT earthing system is characterised by having a perceived safe but identifiable fault condition with a 2nd
fault being capable of isolation:
1. 1st fault condition: a high impedance fault generating up to 1amp but widely regarded as not being
dangerous. It represents the ‘Bird on a Wire’ concept.
2. 2nd fault condition: is very dangerous, with the current rising between the two faults up to 20,000 amps.
A 1st fault condition can lay undetected for long periods of time without causing harm to individuals or damage
to equipment. Conventional RCD’s are unable to detect this condition as there is no imbalance of fault current
for the RCD to detect.
A 2nd fault condition may be a high impedance fault, which can restrict the fault level to a low, but potentially
lethal level (eg: between 70mA and approximately 10A). Not only will the RCD fail to detect this fault but the
MCB will also not trip as this fault is seen as a load.
A safer solution
The RVD places an additional layer of electrical protection to the existing features of conventional RCD’s
together with a unique ability to detect voltage leakage to frame. This enables the device to detect and cause
isolation of the ‘Non Dangerous 1st fault condition’ when operating in an unearthed system and as the 1 st is not
generally regarded as being dangerous, represents a significant enhancement to electrical safety for users of
portable generators and other equipment such as power inverters. A fault detection level of 8mA and isolation
operation within 20mSec, significantly enhances the performance of the RCD without unduly increasing
nuisance tripping. RVD’s when installed within vehicular situations or power-boards, provide a perfect solution
to the many shore power/ generator, inverter power applications. Installation of the RVD to existing or new
installations is a very simple task.
Screened Electrical leads
Some regulating bodies have recently suggested the use of conductive screening on all electrical leads (and
cables) used to distribute power from mobile generators. Particularly harsh environments such as mining,
construction and wet areas can benefit significantly from their use.
Screened leads can be used to enhance the safety of the protective system. With the screen is connected to the
earth cable at the point closest to the protective device, it acts as an additional conductor of any leakage in the
line that is not to frame. This enables isolation at any point where the lead may be damaged such as when the
active or neutral is exposed. Hence screened leads provide a valuable additional layer of protection which
reduces the risk of potential injury or damage to equipment.
The above figure shows that damage on an exposed conductor in a screened lead will bring the screen into
contact with the conductor. When the screen is bonded to the earth the Universal RCD will recognise the
voltage leakage fault and isolate the system.
Conclusion
In situations where achieving an effective earth path is impossible or impractical, the combination of ‘protective
electrical separation’ with RCD and the RVD’s unique voltage detection, provides the most effective form of
protection. The RVD when used on an unbonded generator provides clear benefits and a solution to the long
argued debate of whether it is best to earth or not. The benefits are obvious with higher levels of personal
protection and ‘peace of mind’ for users together with cost benefits achieved from better efficiency in
deployment, less damage to equipment and the possibility of reduced insurance premiums. Further benefits can
be experienced through utilisation of screened leads which enhance the levels of electrical protection that can be
achieved.