Electrical Safety In Laboratories And Small Scale Production – Regulation 411

Many laboratories and small scale manufacturing facilities such as in the food process industry, use equipment connected to a 230V AC electrical supply via plug and cable. These installations are covered by the requirements of the IET Wiring Regulations (BS7671), as specified by the Electricity at Work Regulations (EWRs). Amendment 3 introduces new requirements relating to RCD protection.

The Electrical Safety Council have estimated, that around 2.5 million people receive an electric shock each year, of which 350,000 receive a serious injury.

Amendment 3 of BS7671 introduces new requirements relating to RCD protection for sockets outlets irrespective of the location e.g. office, lab, manufacturing area etc. RCD’s provide additional protection when a primary protection method fails. Ensuring they work when called on to do so, could mean the difference between life and death.

Scope

The EWRs make reference to BS7671 for Installations/electrical equipment as mentioned above. Amendment 3 of BS7671 Regulation 411.3.3 now requires as a minimum,  30 mA RCD protection for “ALL” socket outlets up to 20A (chapter 41: Protection Against Electric Shock). There are 2 exceptions allowed under this regulation:

• “Documented risk assessment determines that RCD protection is not necessary”.

Justifying the use equipment connected to a standard UK supply without RCD protection needs careful consideration i.e. EWR 2.35 states that “You should always consider the conventional public electricity supply voltage of 230 V ac as potentially fatal”.

• “Suitably identified socket-outlet provided for connection of a particular item of equipment”.

HSE Guidance HSR25 3^rd Edition Oct 2015(EWRs)  

Where sockets outlets are not adequately protected by RCDs, steps must be taken to ensure that “ALL” staff are aware of the increased risk of electrocution either through direct or indirect contact. They must be suitably trained and supervised – Regulation 16 places a duty of care on Employers “No person shall be engaged in any work activity where technical knowledge or experience is necessary to prevent danger or, where appropriate, injury unless he possesses such knowledge or experience…. “.   Likewise individuals must not put themselves at risk or others as a result of their actions. i.e. don’t do it until you understand what you are doing.

Design for Safe and Reliable operation

When designing electrical installations for the connection of complex electrical equipment (Non Linear Loads), the designer/electrician must take in to account the Type and characteristics of the RCD required.

It may be necessary to install specific items of equipment with high operational leakage currents and or specific RCD requirements on a separate feeder circuit to achieve the required level of protection and operational reliability.

Why is this critical to safety?

Many items of equipment such as centrifuges, scanners, ovens, washing, sorting, packing, use power electronics within the control stages exhibiting non-linear load characteristics. This equipment can produce complex leakage and residual currents with high frequency AC and DC components. These currents circulating within the installation, will affect the tripping characteristics of standard AC RCDs used in many installations. This phenomenon is commonly referred to as “Blinding” – see Fig1. Installing equipment on a circuit protected by the incorrect type of RCD, will compromise the protection levels and safety for staff using other equipment connected to the same circuit.

RCD Blinding

In Fig.1^[1]  the Hysteresis curve0 to B3 represents the +ve half of RCD toroid magnetic characteristic, the green area represents optimum operational area, yellow represents the area of increasing magnetic saturation.  A 50Hz residual current IRac, (I) equal to the tripping value sensitivity, produces a magnetic field 0 to B1 for the +ve ½ cycle. The resultant change in this field as IRac passes through zero for –ve ½ cycle induces a

proportional voltage (II) in the trip circuit winding and would result in the RCD tripping. A residual current with +ve biased dc component IRdc > 6mA, passing through the toroid will shift the operating point of the magnetic material on the H axis to the right.  Now if a residual current IRac with the same value as (I) flows the combined current IRdc+ IRac (III) produces a magnetic field B2 to B3. Although the amplitude of (III) is similar to (I) the resultant voltage (IV) induced in the trip circuit winding is lower and not sufficient to trip the RCD. The RCD will not be able to provide the required protection level.

Selecting the correct Type of RCD

Check the equipment manufactures installation and operating instructions. Manufactures of equipment sold in the UK, have to meet Product Safety Regulations and are required to provide information relating to safe installation and use. If this information is not given in the installation instructions, ask the OEM to provide the information in writing, relating to Type of RCD to be used, Sensitivity and Leakage Current. Under PUWER Reg 8, it is an offence to install equipment unless the manufacture has provided the necessary information for it safe use. This information must be kept on file for future reference, leakage currents are required to complete electrical insulation checks on the equipment. If you don’t have the values to hand, you cannot determine if the leakage current value is normal or due to fault; see – PUWER Reg 6.

RCCB limits of operation / refer to manufacture for full data 

Division of RCD Protected Circuits/Protective Conductor Currents

As a rule of thumb for installations with low THD, the combined operational leakage current of the loads connected to an individual circuit should not exceed 40% of the RCD sensitivity. To reduce nuisance tripping, as THD increases the % loading for the RCD sensitivity needs to reduced pro-ratter. If the leakage current of an individual item of equipment is >3.5mA<10mA you must use 60309-2 connectors. If the leakage current > 10mA there are number of options, available due to the risks associated with high leakage currents – see Regulation 543.7. Equipment with leakage currents above 10mA is not normally suitable for connection to circuit protected by the correct type of 30mA RCD. In this case the use of RCDs will be for fire protection only – see Regulation 532. Where it is necessary to limit the risk of fire in equipment and or wiring systems due to earth leakage / fault currents, RCDs and insulation monitoring devices can be applied. However under the wiring regulations RCMs cannot be used in place of RCDs – see Regulation 411.1. Individual items of equipment with leakage currents >120mA may not be suitable for installation in locations requiring protection by 300mA RCD. The risk assessment for the equipment carried out prior to installation, should identify if this is an issue for the position and location of the equipment. Remember RCDs installed on feeder circuits must be compatible with any RCDs installed upstream of the feeder – see Fig 2

Example of RCCB selection and circuit / load segregation for safety (Reg 531.2.4)

RCDs must operate reliably when other means of protection have failed. Installing / Signing-off an installation which is incorrectly protected, could increase the risk of serious injury from electrocution and or fire.

This article refers to electrical installations covered by BS 7671: 2008 incorporating Amendment No 3. Refer to the IET for the latest copy of 7671 for guidance on the selection and use of RCCB’s. For guidance on UK Safety Regulations refer to the HSE Government web site. For further reading on RCDs from independent sources refer to the BEAMA and Electrical Safety Council’s WEB sites. For information on different Types of RCD please refer to the Doepke Web site http://www.doepke.co.uk/rcd/rcdR.html

 Chaz Andrews – Technical Manager, Doepke UK Ltd        www.doepke.co.uk