by Marcus Birkin, Field Service Engineer
This article is aimed at being a guide to the UK regulations both current: BS7671:2008+A3:2015 (aka “17th Ed”), and future: BS7671:2018 (aka “18th Ed”); with relation to power control in the entertainment and architectural environments.
Although I will make reference to UK requirements, it can equally apply to the rest of Europe, as BS7671 is derived from the European Harmonised Document HD 60364, which in turn is derived from the International IEC 60364.
This article refers to CE versions of products, with a focus on UK/EU regulations
As always this article’s content should be treated as advice, used only in conjunction with a suitably skilled (electrical) individual, familiar with your particular design and regulatory requirements
Additional protection is used in select situations where an additional means of protection against electric shock, along with the standard (primary) protective measures, is required.
The most common, and normally best, way to achieve this is by the use of Residual Current Devices (RCDs).
In the event of an earth fault exceeding the prescribed earth fault current (I(delta)n), RCDs automatically disconnect the supply, thus preventing harm to individuals.
The use of fuses, circuit breakers, and other Automatic Disconnection Systems (ADSs); are primarily for fire prevention.
The use of an RCD in final circuits is generally well understood, however there are some common misconceptions misunderstandings, and mistakes; particularly in our industry.
When are they required?
In final circuits involving a socket rated at 20A or lower [17th Ed 411.3.3], or from the 1st January 2019: 32A or lower [18th Ed 411.3.3]; are obliged to have an RCD that must disconnect in less than 300ms in the event of an earth fault of 30mA or greater.
There are situations where an RCD can be omitted, however these require the use of a documented risk assessment and method statement.
Note that the 18th Ed removes the exception “for a specific labelled or otherwise suitably identified socket-outlet provided for connection of a particular item of equipment”.
Distribution circuits have additional requirements, this is not covered here; however in our industry these are generally only a concern with temporary power installations and details on this can be found in section 4 and 7 of BS7671, as well as BS7909.
Most RCDs in domestic installations are of type AC. That is to say they are sensitive to an earth fault that is purely AC in nature.
This is fine for a simple load, for example a toaster or electric kettle.
However loads controlled by a semiconductor (E.G. A Source 4 connected to a Sensor dimmer), where one of the two back-to-back SCRs fails either on or off would not present a pure AC wave form; but a pulsating DC waveform. .
The use of a type AC RCD in this situation may result in the RCD coil becoming saturated and failing to action in the event of a fault.
Type A RCDs are designed to tolerate pulsating DC or partially DC waveforms.
All RCD protected Sensor modules, and other factory fitted RCD protected ETC dimming solutions; are fitted with type A.
For those of you involved in variable speed hoists, utilising the use of inverters (For example our Prodigy or Vortex hoists), another type of RCD would be required. This is type F, these are sensitive to multi-frequency fault current.
The 18th edition includes a table of RCD types [Annex A53, Figure A53.1]. This has been expanded particularly for the EV car market; but domestic semiconductor controlled devices, such as washing machines, may require special types.
As the range can be quite baffling, it is advised that you contact the manufacturer of the current consuming device at the design phase.
Incidentally, for those of you that also do house bashing - the 18th Ed now requires the use of RCD on all domestic lighting circuits, so don’t forget about those dimmer switches!
Another important design decision is the location of the RCD in the circuit.
It might seem obvious to place them in the distribution board at the origin of the final circuit, however I have seen dimmer installations where the RCD is an afterthought, and the RCD installed after the dimming control.
The RCD must be installed before the dimming control circuit - this includes the use of inline/plugin RCDs on luminaries.
RCDs installed after the dimming control circuit are unlikely to action in the event of a fault.
Periodic testing of installations should be performed at the inspection interval denoted by the designer, previous inspector, or as risk assessments require.
This can be as frequently as every year (schools for example) or anything up to 10-15 years for low risk installations.
But any and all RCDs functionality should be regularly and frequently tested, by an instructed, or suitably skilled person.
BS7671 [17th Ed 514.13, 18th Ed 514.12.2] states that appropriate signage be erected at the distribution board; instructing the operator to test, using the local T button, every 6 months.
This signage is often overlooked.
It is advisable during handover to train the client, as well as include this detail in all O&M manuals.
Incidentally, another often overlooked signage is the notice of high protective conductor current [514.16], as you will most likely find the distribution circuit of a phase angle dimmer requiring high integrity Circuit Protective Conductor (CPC).
One aspect that I often get asked is “how many devices can be connected to this circuit”.
There are many contributing factors to address here: inrush and design current, power factor, etcetera; and further information regarding this can be found in the technical articles available on toolkit.
But with respect to earth leakage and RCDs the answer is fairly simple:
An RCD should only be loaded with devices statically leaking up to 25% of the I(delta)n.
So for example an RCD rated at 30mA can have up to 7.5mA of nominal leakage.
A luminaire (Class 1, with plug, drawing less than 4A) can be considered to have 2mA (r.m.s.) of leakage (BS EN 60598-1:2015), however as ETC devices also conform to Canadian requirements our devices can be considered to have only 1mA of earth leakage!
The reason behind this 25% rule is due to the RCDs actual trip current being between 0.5I(delta)n and 1I(delta)n. I.e. Anywhere between 15-30mA.
The only requirement is they must trip at I(delta)n, and must not trip at 0.5I(delta)n.
Having a static value of 0.25I(delta)n allows for intermittent peaks of earth leakage during certain operations, for example at turn on.
I hope that this brief article is enlightening and helpful. Should you have comments or questions, please feel free to contact your local ETC Technical Service office.