Electrical safety – FAQ

 

Topics –

 

~    My equipment needs testing. Who do I ask?

 

~    Extension leads – how long can they be, is daisy chaining allowed?

 

~    What size fuse do I need?

 

~    Test intervals – How to decide

 

~    Useful calculations electrical principles and data relating to electrical safety

 

~    Recent Electrical Safety Issues & Information.

 

My equipment needs testing. Who do I ask?

 

Firstly, where are you using the equipment? – Depending on the location and department, please contact one of the following people for assistance:

 

Location:                                               Contact(s):

 

Teaching Labs, AB Level 4:              Richard Batten      Tel 6063                 R.Batten@surrey.ac.uk

                                                                Stephan Brash      Tel 6063                 S.Brash@surrey.ac.uk

(Teaching Lab Manager)                    Dave Fishlock       Tel 9132                 D.Fishlock@surrey.ac.uk

 

CVSSP AB Level 5:       Simon Alcott       Tel 3957                 S.Alcott@surrey.ac.uk
 
EARS AB Level 5:                               Bob Doran             Tel 6107                 R.Doran@surrey.ac.uk
                                                                Richard Batten      Tel 6063                 R.Batten@surrey.ac.uk

 

Fab Lab (18) AB Level 4:               Vernon Power       Tel 6094                 V.Power@surrey.ac.uk

                                                                Alex Royle             Tel 6097                 A.Royle@surrey.ac.uk

 

Computer Labs BB Level 4/5:         

 

Nodus Centre:           Adrian Cansell      Tel 9847                 A.Cansell@surrey.ac.uk

                                                Mark Browton      Tel9852                  M.Browton@surrey.ac.uk

 

ATI - Computing Services:                Daryl Patto            Tel 6087                 D.Patto@surrey.ac.uk

 

ATI - SCRIBA Labs:                           Dean Mansfield    Tel 9828                 D.Mansfield@surrey.ac.uk

                                                                Gary Strudwick     Tel 2715                 G.Strudwick@surrey.ac.uk

 

 
For all remaining areas of Electrical Engineering, Computing and Mathematics, &
safety advice or technical assistance in connection with electrical safety, please contact:
 
Charles.W.Murray                    Tel 9839                                  C.Murray@surrey.ac.uk
 
To book or request electrical safety test, please contact:
David Dryden                           Tel 9839                                  D.Dryden@surrey.ac.uk
Personal items must also be tested if they are to be used within the school, Take the item to be inspected or tested to Room 04AB04 (Electronics/Computing/Maths) where someone will be able to carry this out for you.
 
For all enquiries relating to the Physics department, please contact the school health and safety advisor –
 
Chris Stringer                           Tel 9416                                  C.Stringer@surrey.ac.uk
 
IMPORTANT – When purchasing new equipment please contact:
 
Richard Clarke                         Tel 9839                                  R.Clarke@surrey.ac.uk
 

 

 

 

Extension leads – how long can they be, is daisy chaining allowed?

 

The IEE code of practice specifies the following maximum lengths for extension leads based on the cable size:

 

CSA of Cable              Maximum length

 

1.25mm2                      12 Metres

 

1.5mm2                        15 Metres

 

2.5mm2                        25 Metres – Note 2.5mm2 is too large for a standard 13A plug!

 

Generally most extension leads use cable with a CSA of 1.5mm2, however there are some leads with smaller cables in service. As a general recommendation all extension leads (unless essential) should be restricted to 5m or less in length.

If the lead is too long – DO NOT COIL, obtain an extension lead with a shorter cable.

 

Daisy Chaining.

 

Daisy chaining is to be discouraged. If the lead is too short – use a single, longer lead noting the limits on length. If more socket outlets are needed consider changing the lead for one providing more outlets. If daisy chaining is unavoidable then the following guidelines must be adhered to: The total length of cable from the supply point to the furthest socket outlet on the chain must be less than 90% of the maximum permitted length for the cable of smallest CSA used in the leads, based on an equivalent single extension lead. No more than 3 leads in any chain (end to end) however, star connected arrangements are permissible, i.e. one 4 way lead can feed 4 extension leads, each feeding 4 more leads – this reaches the limit of three leads in any given chain.

 

 

 

 

 

 

What size fuse do I need?

 

Fuses for standard 13A plugs, need to comply with BS1362, and are available in the following sizes:

 

1A       2A       3A       5A       7A       10A     13A

 

The sizes most commonly used are 3A, 5A, and 13A.

 

The following table gives examples of fuse ratings usually encountered for various items of equipment:

 

Type of Equipment

Fuse Rating

Conventional computer monitor up to 17”

3A or 5A

Conventional computer monitor >17”

5A or 7A

LCD computer monitor (all sizes)

3A

Computer – Main unit, not supplying VDU

3A

Computer / Workstation – supplying conventional VDU

5 or 7A

Computer / Workstation – supplying LCD VDU

5A

Printer - desktop

3A

Printer large, colour laser

5A

Auxiliary items – speakers, external drives, scanners etc

3A

Laptop computer

3A

Test equipment – small, bench-top

3A

Large test equipment including modular systems

5A or 7A

Test Equipment – Rack mounted systems with distribution

Up to13A

Room Heater

10A or 13A

Kettle

10A

Fan

3A

Air conditioner - portable

5A

Soldering Iron

3A

Small (desktop) photocopier

5A

Large photocopier

7A

Refrigerator / Freezer (for food storage)

5A or 7A

 

 

For any item of equipment where the lead is detachable – for example, uses an IEC lead, the maximum fuse rating must not exceed 7A unless the connector is clearly marked (10A) and the cable is less than 1.5 metres long with a CSA of 1mm2 or greater.

 

For items not on the list, please seek the advise of your nearest electrical safety officer, or refer to “Useful Calculations” (see below).

 

Any items fitted with a 13A fuse – including extension leads, must only be plugged into a socket outlet (wall socket) and not into an extension lead.

 

 

 

 

Test Intervals – How to Decide

 

The following table specifies general test intervals based on typical use. Ultimately an assessment needs to be made based on how an item is used in order to determine the most appropriate test interval.

 

As a general rule, items which are frequently moved or where the risk of damage to the item is high due to the environment in which they are used must be inspected & tested frequently.

 

Items, which are unlikely to move and not subject to fatigue likely to place stress on the structure & electrical parts, can be tested less frequently.

 

Under no circumstances must it be assumed that an item of equipment when purchased new will be exempt, because the manufacturer tested prior to supply. All incoming equipment is subject to electrical testing from new, this ensures that any faults are detected immediately and rectified before they can present a hazard.

 

 

Type of Equipment

Test Interval

Computer monitor

3 to 4 years

Computer / Workstation

3 to 4 years

Printer - desktop

3 to 4 years

Printer large, colour laser

3 to 4 years

Auxiliary items – speakers, external drives, scanners etc

1 to 3 years *

Laptop computer

3 to 4 years *

Test equipment – small, bench-top

1 to 3 years

Large test equipment including modular systems

1 to 3 years

Test Equipment – Rack mounted systems with distribution

1 to 3 years

Room Heater

1 year *

Kettle

1 year *

Fan

1 year *

Air conditioner - portable

1 year

Soldering Iron

3 Months – 1 year *

Small (desktop) photocopier

1 year

Large photocopier

3 to 4 years

Refrigerator / Freezer (for food storage)

3 to 4 years

Power tools (drill, jigsaw)

3 Months – 1 year *

Extension lead - office

1 year

Extension lead - portable

3 Months – 1 year *

 

Additionally items marked * must be inspected by the user prior to use.

 

 

 

 

 

 

 

Useful calculations electrical principles and data relating to electrical safety

 

Fuse basics.

For portable items of equipment, the regulations specify that a protective device (fuse or circuit breaker) should operate within 0.4 seconds in the event of a serious fault. A serious fault is defined as a fault where the current flowing in the protective device exceeds 5 x its current rating. BS1362 fuses fitted in 13A plugs are designed to meet or exceed this requirement.

 

To ensure this requirement can be met, the circuit resistance under fault conditions needs to be sufficiently low, to allow sufficient current to flow through the fuse with the lowest supply voltage present (worst case conditions) which is 184V in the UK, (230V–20%). The IEE regulations specify that the supply impedance at a socket outlet (Zs­­­) shall be less than 2.5 ohms (Based on Table 45A1 IEE reg’s).

 

Fuse Rating

1A

2A

3A

5A

7A

10A

13A

5x FuseRating

5A

10A

15A

25A

35A

50A

65A

Max loop impedance

 

34.3 ohms

15.9 ohms

9.77 ohms

4.86 ohms

2.75 ohms

1.18 ohms

0.33 ohms

Max ZE

17.1 ohms

7.95 ohms

4.88 ohms

2.43 ohms

1.37 ohms

0.59 ohms

0.16 ohms

Max ZE /4

4.27 ohms

1.99 ohms

1.22 ohms

0.61 ohms

0.34 ohms

0.15 ohms

0.04 ohms

Test limit for ZE

0.5 ohms

0.5 ohms

0.5 ohms

0.1 ohms

0.1 ohms

0.1 ohms

0.1 ohms

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The limits for earth loop impedance are specified in the bottom row of the table, when testing items of equipment these limits are used. It is however important to consider the maximum limit for ZE (4th row) above when extension leads are used, since if 3 leads are daisy chained, including the equipment, the combined maximum impedance can be as high as 0.4W. For this reason, any item of equipment fitted with a 13A fuse MUST be connected directly to a socket outlet and not via an extension lead.

 

 

Recent Safety Issues, notices and concerns etc.

Index:

1) Safety note – RS / MARBO Plug-tops,        Issued              17-02-2003

 

 

 

 

 

 

 

 

 

Important Safety Note  Issued 17-02-2003

 

Topic: Electrical Safety – Plugs.            Issued by C.W.Murray  - Electrical Services.

 

This note concerns all moulded, hard rubber plugs supplied by RS or MARBO.

 

Under certain circumstances – the top cover can come away from the body of the plug when removing the plug from a socket.

 

Plugs affected: Any carrying the name “MARBO” or “RS” on a separate moulded panel located in the top of the plug – see illustration below.

 

Action required.

1) Always ensure that the supply is switched off at the socket before removing a plug. 

 

2) Examine any plugs that fit this description for any cracks or other damage in the area circled. If there is any visible damage or any signs of stress please carefully unplug and arrange to have the plug replaced.

 

3) To all electrical safety testers - Replace any plugs fitting this description while carrying out routine safety testing as a precaution.