Removing the MEN link
The MEN link is a mandatory part of the “main earthing system” for any installation set up for supply from the MEN distribution system. And when we set ip an alternative supply system, we don’t touch it. So why remove it in this case?
We need to march the earthing system of the installation to the earthing system of whatever type of generator will be used for the emergency supply. In normal circumstances, the only type of genset that is permitted for connection to an installation is one with an isolated output.
In order to safely use any other type; the installation’s MEN must be removed.
Additionally if there are any other N-E links within the installation, they also need to be removed.
If there’s no removable link – for example an older installation with a single busbar for both neutrals & earths; you can’t set up an emergency supply under this TB unless you first upgrade. The best way to achieve this is to add a neutral bar. Doing it this way will mean that there is less risk of removing any protective earthing conductors.
Lastly install a prominent and clearly-worded label advising that the MEN has been removed; including details of any other M-E connections that may have been removed.
With an RCD-protected genset; the installation’s MEN would be in parallel with the N-E link within the generator. That means some of the return load current will flow in the protective earth conductor; and the RCD will trip. Which is no use to anyone. And far better to have the protection of the RCD than not; so the best option is to remove the MEN.
With a centre-tapped genset, it’s far worse. These generators have a winding that puts 115 V between N & E of the output (see below). The MEN acts as a short-circuit for this source which at best means the overcurrent protection that should be on the output will trip. Unfortunately, many if these sets don’t have any protection on this “neutral” side, only on the “active” side. In which case the short circuit is likely to burn out the winding and may well cause a fire.
So again, we remove the MEN. But now we have 115 V between N & E throughout the installation – with no, or only limited, protection. Very dangerous. Which is why the TB requires us to install an RCD (30 mA) between genset and where it’s connected to the installation. If someone receives a harmful shock, the RCD will operate to remove the current quickly, and so save their life.
Testing the RCD
All RCDs Approved for use in NZ are designed to operate on 230 V supply. With a centre-tapped supply source; there’s only 115 V to earth. So in the event of an earth fault, only half the current will flow as would normally be expected. This means that external RCD testers, designed to create a fault of approx 30 mA to flow in the earth conductor (Active – Earth fault); will actually only cause approx 15 mA to flow – and of course the RCD won’t trip. However the integral test circuit works between Active and Neutral; and since this is still 230 V this test will work.
This type is perfectly OK for connection to a normal MEN installation. This is what is used for every alternative supply. However, when creating a set of instructions for emergency generators, they all have to be treated the same. Especially since many of these will not be “permanently” connected; instead being just plugged into. Which means there’s a good chance that generators may be swapped from site to site; including by people who have no training, qualifications, or experience
So the TB’s instructions:
- “do not use an isolated supply (separated supply system) to connect to an existing MEN installation”; and
- “Make sure there’s a connection between the generator earth and the neutral pole of the generator. If there isn’t a connection, install one” are really only for consistency.