Electrical safety – FAQ

Topics –

~ My equipment needs testing. Who do I ask?

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

~ 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 (Z_s) 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 Z_E	17.1 ohms	7.95 ohms	4.88 ohms	2.43 ohms	1.37 ohms	0.59 ohms	0.16 ohms
Max Z_E /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 Z_E	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 Z_E (4^th 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.