What is an RCD and what does it do?

What is an RCD and what does it do?

1.       An RCD is defined, in BS 7671, as: ‘A mechanical switching device or association of devices intended to cause the opening of the contacts when the residual current attains a given value under specified conditions’.

An RCD is a protective device used to automatically disconnect the electrical supply when an imbalance is detected between live conductors.

In the case of a single-phase circuit, the device monitors the difference in currents between the phase and neutral conductors. In a healthy circuit, where there is no earth fault current or protective conductor current, the sum of the currents in the phase and neutral conductors is zero.

If a phase to earth fault develops, a portion of the phase conductor current will not return through the neutral conductor. The device

monitors this difference, operates and disconnects the circuit when the residual current reaches a pre-set limit, the residual operating current (IDn).

RCDs are used to provide protection against the specific dangers that may arise in electrical installations including:

> Protection against indirect contact

> Supplementary protection against direct contact

> Protection against fire and thermal effects

An RCD on its own does not provide protection against overcurrent’s. Overcurrent protection is provided by a fuse or a miniature circuit-breaker (MCB).

However, combined RCD and MCBs are available and are designated RCBOs.

2.     Types of RCDs

RCD is the generic term for a device that operates when the residual current in the circuit reaches a predetermined value.

2.1  Older installations with ELCBs

Historically, two basic types of earth leakage circuit-breaker (ELCB) were recognised by the Regulations; the familiar current-operated type and the earlier voltage-operated type. The voltage-operated type ceased to be recognised by the Regulations in 1981and today, only the current-operated type is recognised. The voltage operated device can be distinguished by its two separate earthing terminals– one for the connection of the

earthing conductor of the installation and the other for a connection to a means of earthing. Such devices were often used on installations forming part of a TT system where the means of earthing was an earth electrode.

The major drawback with the voltage operated earth leakage circuit-breaker is that a parallel earth path can  disable the device.

  

2.2  Recognised devices

RCDs are manufactured to harmonised standards and can be identified by their BS EN numbers. An RCD found  in an older installation may not provide protection in accordance with current standards. The following list identifies the applicable current standards:

> BS 4293 : 1983 (1993)

Specification for residual current operatedcircuit-breakers. (Replaced by BS EN 61008-

1: 1995, BS EN 61008-2-1: 1995 and BS

IEC 61008-2-2: 1990). This Standard

remains current

> BS 7071 : 1992 (1998)

Specification for portable residual current

Devices

> BS 7288 : 1990 (1998)

Specification for socket-outlets incorporating

residual current devices. (SRCDs)

> BS EN 61008-1 : 1995 (2001)

Residual current operated circuit-breakers

without integral overcurrent protection for

household and similar uses (RCCBs)

> BS EN 61009-1 : 2004

Residual current operated circuit-breakers

with integral overcurrent protection for

household and similar uses (RCBOs)

2.3  Characteristics of RCDs

RCDs are defined by a series of electrical characteristics, three main characteristics are:

1. The rating of the device in amperes, I.

2. The rated residual operating current of the protective device in amperes, IDn.

3. Whether the device operates instantaneously or incorporates an intentional time delay to permit discrimination. Such devices are called ‘S’ or Selective.

Devices are manufactured with different values of rated current and rated residual operating current but we will just consider the rated residual operating current of the protective device in amperes, IDn.

3.     Applications

The correct device must be selected for the particular application. Choosing the wrong device could have serious consequences and could result in electric shock or fire.

3.1  Unwanted tripping

Unwanted tripping of RCDs can occur when a protective conductor current or leakage current causes unnecessary operation of the RCD.

An RCD must be so selected and the electrical circuits so subdivided that any protective conductor current that may be expected to occur during normal operation of the connected load(s) will be unlikely to cause  unnecessary tripping of the device (Regulation 531-02-04 refers). Such tripping can occur on heating elements, cooking appliances etc., which may have elements that absorb a small amount of moisture through imperfect element end seals when cold. When energised, this moisture provides a conductive path for increased leakage and could operate the RCD. The moisture dries out as the element heats up. Although not precluded in BS 7671, it is not a requirement to use an RCD on such circuits if other satisfactory means of protection are available.

Providing an RCD with a higher rated residual operating current may solve the problem but the requirements of the Regulations would still have to be met.

3.2  Discrimination

Where two, or more, RCDs are connected in series, discrimination must be provided, if necessary, to prevent danger (Regulation 531-02-09 refers). During a fault, discrimination will be achieved when the device electrically nearest to the fault operates and does not affect other upstream devices.

Discrimination will be achieved when ‘S’ (Selective) types are used in conjunction with downstream general

type RCDs. The ‘S’ type has a built-in time delay and provides discrimination by simply ignoring the fault for a set period of time allowing more sensitive downstream devices to operate and  remove the fault.

For example, when two RCDs are connected in series, to provide discrimination, the first RCD should

be an ‘S’ type. RCDs with built in time delays should not be used

4.     Labelling

Regulation 514-12-02, states that:

“Where an installation incorporates a residual current device a notice shall be fixed in a prominent position at or near the origin of the installation. The notice shall be in indelible characters not smaller than those here illustrated and shall read as follows:”

	This installation or part of it, is protected by a  device which automatically switches off the  supply if an earth fault develops. Test quarterly by pressing the button marked “T” or “Test”. The device should switch of the  supply and should then be switched on to restore the supply. If the device does not switch of the supply when the button is pressed, seek expert advice

 

           5.     Testing

RCDs must be tested. The requirements are stated in the following Regulations:

a. The effectiveness of the RCD must be verified by a test simulating an appropriate fault condition and independent of any test facility, or test button, incorporated in the device (Regulation 713-13-01)

b. Where an RCD of 30mA provides supplementary protection the operating time must not exceed 40 ms at a residual current of 5 IDn.

(Regulation 412-06-02 refers) Tests are made on the load side of the RCD between the phase conductor of the protected circuit and the associated cpc. Any load or appliances should be disconnected prior to testing. RCD test instruments require a few milliamperes to operate; this is normally obtained from the phase and neutral of  the circuit under test.

When testing a three-phase RCD protecting a three-wire circuit, the instrument’s neutral is required to be connected to earth. This means that the test current will be increased by the instrument supply current and

will cause some devices to operate during the 50% test, possibly indicating an incorrect operating time. Under this circumstance it is necessary to check the operating parameters of the RCD with the manufacturer before failing the RCD.

5.1  Range of tests

5.2  Integral test device

An integral test device is incorporated in each RCD. This device enables the mechanical parts of the RCD to be

Verified by pressing the button marked

‘T’ or ‘Test’.

6.     Test Instrument

The test instrument used to test RCDs should be capable of applying the full range of test current to an in- service accuracy, as given in BS EN 61557-6.

This in-service reading accuracy will include the effects of voltage variations around the nominal voltage of the tester. To check RCD operation and to minimise danger during the test, the test current should be applied for no longer than 2s. Instruments conforming to BS EN 61557-6 will fulfil the above requirements. ■

Device Instrument test current setting Satisfactory result

General purpose RCDs to BS 4293 and RCD protected socket-outlets to BS 7288

50% of operating current Device should not operate 100% of operating current Device should operate in less than 200ms. Where the RCD incorporates an intentional time delay it should trip within a time range from 50% of the rated time delay plus 200ms’ to 100 % of the rated time delay plus 200ms

General purpose RCCBs to BS EN 61008 or RCBOs to BS EN 61009

50% of operating current Device should not operate 100% of operating current Device should operate in less than 300ms unless it is of ‘Type S’ (or selective) which incorporates an intentional

time delay. In this case, it should trip within a time range from 130ms to 500ms

Supplementary protection against direct contact IDn ≤ 30mA

Test current at 5 IDn. The maximum test time must not be longer than 40ms, unless the protective conductor potential does not exceed 50V. (The instrument supplier will advise on compliance).

Device should operate in less than 40ms.

I hope this overview will be of service to others, for further information do not hesitate to contact me directly from our website www.safe-electric.com or by E-Mail : - enquiries@safe-electric.com

The real cost of testing Cheap

Unfortunately when you’re trying to compete with a market slowly drowning in 5DW, who can get through the Part P course, and think they know what they’re doing in an office block or a factory, let alone a house, but do not even know what a piece of VIR, Rubber, Lead, Pyro Etc. is no wonder the industry is in such a mess, but they are cheap!

That coupled with using Testing as a loss leader for the past 3 decades, which most of the large companies are all guilty of;  just to get the remedial works, and now you’re left with Large FM companies trying to get you to do testing for £ 4.50 per circuit because they bought the work at £5.50, need I say more.

Testing is not considered important, it is also considered a waste of time and money by many managers and an unnecessary cost, if they can find a way out of doing it or delaying it they will.

And faced with paying £ 8-14 pounds per circuit, for a professional company, subject of course to the type of building or paying £ 4.50 they will always go with the cheaper price, they are only interested in getting the piece of paper and cannot see the Danger in getting it done cheap, because they do not know what is correct.

I lost out to one of these companies a few years ago, they are no longer here, they were cheap one of the building tested, went up in smoke 18 months ago the insurance refused to pay out as there paperwork was considered worthless upon proper inspection, unfortunately 3 people died.

They control over 1500 buildings around the country all done by this company, upon closer inspection, the insurance company have now refused to insure the bulk of their properties until proper inspections are done.

Guess what they are trying to get it done for less than £ 5.00 per circuit, they have not learnt, and obviously there staff and tenants, mean nothing to them, I say make Testing LAW like the GAS and stop the 5DW and the Cowboys once and for all, simples. 

Legal Requirements

The law requires that all business owners, their directors, managers and employees comply with "The Electricity at Work Regulations".

The law simply requires that all electrical systems must be maintained to be safe at all times.

No system or part of it may give rise to danger during either normal operation or under fault conditions.

Regulation 4(2)
"All systems to be maintained so as to prevent, so far as is reasonably practicable, danger"."System" means the wiring of an electrical installation and everything connected to it.

Regulation (29) states:
"It shall be a defence for any person to prove that he took all reasonable steps and exercised all due diligence to avoid the commission of that offence"

Test reports, together with other preventative maintenance documentation will form the basis of this proof and will provide your best defence against prosecution should the need arise.

Duty holders are responsible for setting up adequate maintenance programmes and must assess the needs and frequency requirement of testing, maintaining and monitoring electrical systems and equipment. Generally, the following test intervals may be applied for guidance purposes: -

Electrical installations
(As outlined in 17th Edition I.E.E. regulations)

Type of Installation	Maximum period between inspections
Domestic Commercial Industrial Churches (over 5 years old) Places of public entertainment Petroleum stations Emergency lighting & Fire alarms	10 years rental on change of tenancy 5 years 3 years 1 year 1 year 1 year 1 year Note the word MAXIMUM

Call us today for your piece of mind get it done professionally visit our website www.safe-electric.com and sleep soundly at night knowing you have done your best to make sure you employees and tenants are safe.

Part P, the true cost to the electrical industry

Part P.

When this first came out, like most electricians we hoped it would constitute a level playing field and remove the Cowboys from the industry.

Unfortunately, all it has done is undermining the very industry that most time served electricians loved and valued.

1. It now takes, six weeks to be qualified as an electrician! 
2. Time served electricians like myself who spent 5 years learning, and in my case was not even allowed to install electrical cable's till I understood construction, bricklaying, plastering, carpentry, plumbing, and a whole host of other trade related items to ensure that I could repair the damage I caused when installing.
3. Children can pass exams, but have no practical experience; an example of this, was an 18-year-old young man who asked to join my company as an electrical tester having passed his 2391 – 10, and technically was now competent  to test, so I showed him a board, I have which has various cable types clipped to it with the name hidden by Velcro tab, and asked him to name each one, the only two he could name were armoured, and he couldn't tell the difference between SWA and XLPE and Twin and Earth.

         He had no idea, of the other types of cable on the board.

Here's a short list of just a few of them: space-

1. VIR
2. Rubber
3. lead
4. Pyro
5. FP 200  
6. HORF

Faced with, the government requirement to reduce the number of unemployed people with promises of £50,000 a year for electricians,  their part P qualifications allowing them to do electrical work, in a domestic situation only!

Even this has its problems the basic part P, only allows you to carry out work in a kitchen, bathroom, and conservatory or garden, you need the extension to carry out the work, in the rest of the house.

So the general public, being ignorant of the actual qualifications assume all part P qualified electricians are capable of doing everything, (so much for the advertising).

In general, anyone that has passed a qualification such as part P, or 2391 – 10 is deemed to be competent.

Furthermore, most of these so-called qualified inexperienced people, who don't have public liability insurance efficacy insurance, are not members of publicly recognised trade bodies, such as the NICEIC, don't have proper offices, only have mobile phone numbers and therefore only have very small overheads, have been under-pricing work at such a ridiculously dangerous level, they are in my opinion putting at risk the lives of the general public.

I was called in to test and inspect a property that had been rewired by a part P engineer, who the homeowner could no longer find, after a little research, I found that the person was not registered the homeowner had not checked if they were a member of a competent body and in fact they had disappeared out of the country!

I found 6 x C1 items, 8 x C2 items, and 12 x C3 items; on a brand-new installation, was he qualified YES according to the government he did his six-week course and was a qualified electrician!? did he care about the people in the house NO, did he care about their safety, obviously not, can he now be found No he has disappeared back to Poland, NO is he accountable NO, what can the householder do nothing, except get it put right, at their expense.

But he was cheap!

I have long been an advocate, for the IEE regulations, to be made LAW.

As per usual, the gut reaction of government, has been to Institute a system, that at first appears to be a good idea, it will reduce the number of cowboys in the industry, unfortunately all it has done is instigate a false premise, massively reduced truly competent contractors and instigated a full sense of security to homeowners, but these installers are cheap!

Why is it, that GAS is, is a legal requirement, but electricity is only a recommendation?

Which allows lawyers, fly by night contractors, unqualified individuals, to do what they like; in fact a road sweeper can become an electrician within a day, just by saying he is, and there is nothing legally to say he can't.

Cost-cutting in our industry, has now gone so far that most bona-fide contractors, who carry the overheads and requirements of updating, upgrading, new exams, new certificates, new books and everything else that that entails, can no longer do the work for the prices that the large FM companies insist they can be done for, let alone work for wages, or even less than wages, with their overhead costs

Many clients are now lulled into a false Sense of security, and the paperwork to be honest having seen many of these documents in the past six months, might just as well be used for toilet paper.

They won't stand up in a court of law, insurance companies will not cover you, and they have been a number of cases recently where electrical fires have occurred, and because the paperwork is not up-to-date and in fact is onerous to say the least, they refused to pay out! (And you thought you were insured).

Don't be deluded by cheap prices, reduce costs so-called qualified engineers, cheap testing companies, because the reality is, when it goes wrong, which there are thousands of these occasions every year, you will not be insured, you will lose your home, you will lose your livelihood and you will be put in prison, there are a number of people that already have, with tens of thousands if not hundreds of thousands of pounds of fines, which the un-insured cannot meet, and therefore lose every asset they have!

I see these comments, and many others all over the Internet, yet as per usual the government take no notice, they were fast enough to instigate part P, when an MPs daughter got electrocuted, fast enough to instigate separate governing bodies, who are supposed to be our voice, yet when you asked to speak to the person in charge or talk to the MP they're not interested in listing, you are not big enough, in fact they don't care.

Yet they constantly fall short of making it law, but they make a lot of money, out of the numerous plethora of courses, that take weeks to qualify people to carry out dangerous installations within homes commercial premises and even factories, not the years of installation and practice under supervised experienced engineers that now the minority of contractors have.

Personally for me, one life is far too precious to lose, let alone the number of people that have been electrocuted due to bad DIY, poor installation work and Fred the guys read the DIY book that shows you how to install it, but has no practical experience whatsoever.

For further information please visit our website at: -  www.safe-electric.com