Hi Folks,
I can see that the announcement of the new MF marked Wagobox has caused some interest. Specifically in why the instructions for BS5733-MF are different to BSEN60670-22.
Put simply they are two different standards. BSEN60670-22 is a Junction Box standard whilst BS5733 specifies general requirements for electrical accessories. Also, "BS" (British Standard) document is generally confined to the UK and some British territories, a "BS EN" (British Standard Euro Norm) document applies to many European countries and includes the UK. Consequently there are some geographical/historical differences between the documents, specifically the current ratings for different cable sizes. In BSEN60670-22 the ratings are different to those in BS5733.
So from the perspective of assembling a BSEN60670-22 Junction Box with maintenance free terminals, no change. But to assemble a BS5733-MF accessory then a different set of requirements need to be adhered to.
In effect the challenge for us was to do the necessary analysis and testing to assure ourselves that the Wagobox design was capable or being assembled and installed repeatedly such that the completed accessory would be reliably compliant to BS5733-MF.
(If you are familiar with the Wagobox design you know that the system is very configurable but also that there are several good design features that make repeatability easy to achieve. So completing the necessary work for BS5733 MF was going to take time but was not impossible).
The object of the amended instructions is to provide a route to assembling a BS5733 MF accessory (and not specifically a BSEN60670-22 junction box, though this is not lost in the completed accessory), so as there are differences between the two standards there are going to be differences between the instructions.
The most noticable of these is the catch lock on the Wagobox, BS5733 requires a locking mechanism so this has been added. The second is the new product marking that will be on the underside of the lid which will have all the markings and rating information required by BS5733.
With respect to the de-rating of the terminals. This has been done for two reasons.
Firstly because the test currents in BS5733 are lower than those in BSEN60670-22 (bear in mind that the purpose of the exercise is to assemble a BS5733 MF accessory).
And secondly because the cable clamp of the Wagobox will not work with 4mm Flex eg the 222 terminals are rated at 32A with 4mm flex and that size of cable will not fit into the cable gripper, therefore from an accessory point of view it would make no sense to indicate the use of a cable size that it is not physically possible to install. Consequently the BS5733 marking only indicates that which is relevant to a completed accessory, not what is possible to achieve with the terminals in other circumstances.
With respect to max aggregate current. An accessory that is supplied with moulded in terminals will also have a maximium aggregate current and this will work in the same way that it works with the Wagobox. For a junction box with a fixed number of terminals this limit will be a function of the current rating and the number of phase terminals, so can be fixed by the manufacturer and is not of any concern to the installer. For the Wagobox the configuration of the terminals is done by the electrician and so for the Wagobox this could be a concern.
Limiting the maximum aggregate current limits the amount of heat that can be generated within the enclosure. Each phase terminal (Lives, Neutrals and Switch lives) within an enclosure generate heat as there will always be a small resistance at any junction. The CPC (earth cable) will not be carrying any current in normal operation as the CPC only carries a current in the event of a fault and consequently does not generate heat in normal use.
(Note: in normal operation when the Wago terminals are operating within their current rating the amount of heat generated is small and the temperature rise within the Wagobox is < 40 deg C as required by BSEN60670-22 even when running at the maximum rated current of the terminals.)
So why is there an issue with generated heat for BS5733 Maintenance free accessories? In BS5733 there is a performance test that requires the accessory to be run at **twice the rated current** for 90 minutes. To pass this test the temperature between the accessory and the mounting surface measured at the worst position must not exceed 90 deg C at the end of the test period. Also there should be no damage to the accessory and cables that would impair it's continued safe operation.
(Quick reminder of the power equation: P=I2R. ie the amount of heat generated by a terminal increases exponentially with the increase in current!)
To assemble a Wagobox the electrician has to be aware of the maximimum permited aggregate current and how to apply this to the Wagobox when configuring the circuit. Compliance with this rating will ensure the assembled Wagobox will easily pass the above long duration overload test. Please be assured that this additional rating factor is not as complicated to adhere to as it might first appear. I will create a set of worked examples and post them on the website.
It is worth noting that other MF accessories have also de-rated the terminals from 32amps to 20 amps just to emphasis there is nothing sinister in doing this!
In summary, the Wagobox system can be used by an installer to create a BS5733-MF accessory providing the BS5733 requirements are adhered to and installation instructions are followed.
I hope this helps to answer any concerns but please do feel free to call if you would like to ask any further questions.
Many Thanks,
Ray.
Connexbox Ltd
01353 666011
