Powering the Workshop archive

Tuesday 11 May 2010

Alan Holtham switches you on to the laws of workshop electrics and puts the cases for single and 3-phase supply


I need to make it clear right from the start that I am not a qualified electrician. This article is a distillation of my accumulated practical knowledge from many workshop wiring projects coupled to some basic research and is intended as an overview to workshop electrics rather than be a definitive guide.

And here lies the first problem. Until the introduction of part P of the building regulations in April 2005 you were more or less free to install and modify the electrical system in your home and workshop. Part P was designed to try and reduce the number of people being electrocuted by their own incompetent wiring.

Legal requirements

In England and Wales the regulations now specify that a competent person, i.e. a qualified electrician, should install and certify your workshop wiring. He or she will ensure that the installation is installed and tested to the standard required by BS 7671.

You can be fined up to five thousand pounds for non-compliance, and be compelled to remove totally the offending work or have it brought up to the required standard. If you sell the workshop the purchaser will need to see the correct certification.

Part P does specify some non-notifiable work such as changing a socket, adding lights or replacing a damaged cable, but even in these cases there are some exclusions that have to be carried out by an electrician. If you have any doubt, check the part P regulations and also the IEE wiring regulations 17th edition BS 76 71, 2008.

If you consider that you are capable of carrying out the work yourself, this is sometimes possible but then you will have to notify your local building control office, who may then inspect the work themselves or employ an improved inspector to issue a completion certificate.

Modify the installation yourself and saying nothing could leave you open to problems with any subsequent insurance claims if the electrics are not legal.

Employer implications

If you think part P is onerous, the situation changes dramatically if you start employing because you come up against a whole raft of other regulations. In particular, your electrical installation must comply with the Health and Safety at Work Act 1974, the Management of Health and Safety at Work Regulations 1999, the Electricity at Work Regulations 1989, the Workplace health, safety and welfare Regulations 1990 and the more recent Provision and Use of Work Equipment Regulations 1998, PUWER.

In reality, however, basic common sense will pretty much ensure compliance with the regulations and there is inevitably a lot of overlap between them. For instance, compliance with the Electricity at Work Regulations 1989 should ensure compliance with PUWER 1998, so it is not quite as demanding as it looks at first. Interestingly the Health and Safety at Work Act 1974 puts the duty of care on both the employer and the employee to ensure the safety of all workers using the premises and this includes the self-employed.

Portable appliances

The PUWER 1989 regulations are particularly wide ranging, but much of the content concerns the use of portable appliances and to my mind the implementation is something of a grey area. As the definition of portable is any appliance connected to the mains by way of a flexible lead and plug that covers virtually all your workshop machines as well as power tools. You can pay to have all your portable tools PAT tested, and whilst you may wish to do this to comply with the Electricity at Work Regulations, or as an insurance requirement, there is in fact no specific regulation that demands PAT tests to be carried out or how often. It is often left to the person responsible for the safety of appliances to draw up a regular system of maintenance and testing, particularly as they are liable to prosecution if any fail to conform to all the other regulations.

Explaining the physics

Electrical power is supplied in the UK in the form of single-phase 240V or 3-phase 415V alternating current AC. Quoted values are often given as between 220 and 240V, the result of harmonisation within the EU where all electricity supply now has to be 230V plus or minus 10 per cent. We actually specify 230V plus 10% minus 6% in the UK, so in true EU fudge fashion this means that we can still supply 240V. In reality the mains voltage in your workshop should be around 230V, but I have usually found it to be 240V.

Single-phase current

Most workshop machines are supplied as suitable for single-phase electrics and will plug into a standard 13-amp socket, but heavier and more industrial models may be offered as three phase.

As the name suggests the electrical energy in an AC supply flows in one direction around the circuit and then reverses, and for our supply this happens 100 times per second, giving a frequency of 50 Hertz 50Hz. This change in direction is not instantaneous but occurs smoothly in a sinusoidal waveform.

The electrical energy is not at the level of these peaks for very long relative to the time between them, and your 230V workshop supply is actually what is called the Root Mean Square value RMS of the true peak mains, which in reality is around 325V. In other words 230V is the average power and not the peak power, but we all use RMS values for voltage and current when talking about mains supply.

A single live voltage at 230V RMS is called single phase and is supplied from a standard domestic 13-amp plug or blue 16-amp ceeform connector using a single live wire, a neutral and earth.

3-phase current

Three-phase power on the other hand is supplied via a red 5-pin connector consisting of a single earth and neutral and three live voltages at different phases.

Each of these phases has an RMS voltage of 230V and the waveforms are spaced 120 degrees apart. Because each of these phases has the same sinusoidal form, they never all peak together so if you add the three of them together at any moment they total 415V, the line voltage.

Incidentally, most 3-phase motors will only require connecting to the three live wires. There is no neutral as the load is balanced over the three phases. However, a few bigger machines may have some sort of single-phase control motor so it is better to install a neutral connection as well.

Single phase versus three phase

Most domestic premises are supplied with just a single-phase supply. This is perfectly adequate for the supply of power up to about 11kW which you will probably not exceed in a home workshop.

There are, however, some significant advantages to using three phase.

1. It allows for much higher power levels, important if your workshop starts to grow, both in terms of the number of machines and their individual power requirements

2. Three-phase motors are smaller and lighter than their single-phase equivalents and they will start directly without the need for bulky capacitors and internal switches, and will also power up against higher torque loadings

3. The power delivery is much smoother and also allows for the introduction of variable speed via an inverter

4. Finally, good-quality 3-phase machines can be bought second-hand from auctions and closing-down sales for considerably less than single-phase equivalents as they have a much more restricted market

Buyer beware

If you are looking at second-hand machines though there is a possible area of confusion because the plate on a single-phase motor will clearly state the input voltage as 240V. However, most newer 3-phase motors are what is called dual voltage which means they can run off 415V or 240V but on three phases. It does not mean that they are single phase.

Unfortunately very few domestic premises have a 3-phase supply. You can usually check this by looking at the number of incoming wires. Single phase will normally be supplied in three wires depending on the earthing arrangements whereas 3-phase will usually be five wires, though just to confuse things modern suppliers now have all the cables bundled together into a single heavy strand.

You can also look at the number of meters as 3-phase requires a meter for each phase. If there is only one meter it is single phase.

Cost implications

Installation of a 3-phase supply is likely to be considerable. If you are in a built-up area you will find that most streets are fed with a 3-phase supply, each house taking an alternate single phase to try and balance the load over all three. If this is the case you may be lucky and the cost of wiring into your workshop will not be too prohibitive.

In more rural situations the 3-phase supply may be much further away and the cost then becomes significant. Also, be aware that the price tariff for units used on 3-phase is normally higher as you are then classed as commercial.

Phase converters and inverters

An old-fashioned phase converter or a relatively modern inverter which has speed control benefits appear to defy all logic by producing three phases from a single-phase input. The market has recently become slightly confused with the introduction of digital invertors which offer both speed control and phase conversion at the same time, but there appears to be some debate about the legality of these units so I will stick to discussing proven technologies.

Phase converters

A phase converter is a completely separate unit. In theory you plug one end into the 240V mains and the other into your machine, without the need for an electrician. An inverter on the other hand needs to be wired directly into the machine so it becomes a single-phase machine.

Phase converters are available in two different forms, static and rotary, both housed in big heavy boxes that must be sited near the machine with plenty of ventilation. They both work well provided the proposed use is carefully matched to the right converter, and will run 3-phase motors at virtually full power at single phase. It is not really a one size suits all scenario and there is quite a bit more to it, so you will need to get some technical advice from the converter manufacturer before you buy.

Static converters

The static variety of converter is the simplest and cheapest but there is the potential for the machine motor to be damaged by overheating if the capacitance of the converter is not carefully tuned to it.

In reality static converters do not quite produce three proper phases. It is more like two plus an artificial phase. This deficiency becomes more pronounced if the machine is stopped and started continually and for this reason a static converter is best suited for a single, smaller machine whose use is not too intensive.

If you start putting several motors onto a static converter, as in a machine with an extractor, then these capacitance problems become exaggerated and you also have to take great care to start the motors in the correct size order.

Rotary converters

Rotary converters are more expensive as they use an internal motor to generate the missing third phase but they do produce an output whose quality is equal to mains three phase, but it can be a bit noisy.

The other disadvantage of a converter is that there is no option for speed control because they fix the 3-phase output at 50Hz. Neither can they be programmed for a soft start and stop.

However, they will power several different machines at once and are less likely to cause motor damage, and you can start motors in any order. They are also available for really big motors up to a maximum of 30hp provided the single-phase supply is good enough.


The third option is an inverter. This produces a 3-phase supply at 240V and also offers electronic speed and direction control. Inverters start at below one hundred pounds , but they do have to be hard wired into the motor which will require some modification to the machine and its existing switchgear, so it is not really something you can do yourself and is also notifiable under part P.

Machines designed from scratch can incorporate the inverter and its controls very neatly. You can also buy remote control pendants for them allowing you to mount the controls just where you are working, something that is not always possible with a converter.

Inverter disadvantages

The main disadvantage of an inverter is that unless you set the supply to run at maximum frequency, i.e. 50Hz, you will get less power out than via a converter, and this can reduce torque at low speeds. Also with an inverter you will only get full power using a dual voltage 415V motor wired for 240V. Using it wired for 415V will reduce the output considerably.

Be aware too that when using a variable-speed inverter at low speeds the effect of the motor cooling fan is reduced, increasing the possibility of the motor overheating, so take care if you are running the machine for any length of time at low speed.

The other slight disadvantage of an inverter is that they are less robust than a converter, their delicate electronics are susceptible to vibration and dust and they are not available for larger motors.

Breaker device effect

Because of the way they operate and can affect the single-phase network, inverters are normally only supplied to support 3-phase supply up to 3kW. Also, if your workshop is supplied via an ELCB, RCD, or RCCD breaker device you may find that an inverter causes it to trip, in which case you can try changing the breaker type or it may have to be wired via a fused switch with a permanent earth.

Dual-voltage motors

If you want to convert a 3-phase machine to a single-phase one with speed control and are buying a motor from scratch you can overcome the power problem and low speed torque by buying a dual-voltage motor 240/415V with at least a half horse power more than the original.

Woodworkers Institute

Tagged In:

Alan Holtham , Power , Workshop , Shop Talk , electrics

The Cost Inc Vat

1.0hp static phase converter - £235
1.5hp rotary converter - £440
0.75 hp inverter - £105

Useful Links

Transwave converters
Tel: 0121 7084522
Building Regulations
BRE Certification Ltd
Tel: 0870 609 6093
British Standards Institution
Tel: 01442 230442