Consumer units or CU are generally replaced due to different reasons. This may include replacement if the present unit fails to fulfill the needs of BS 7671 where there isn’t any RCD protection or there isn’t any additional capacity in the present consumer unit to connect with other circuits. Replacing the unit is certainly a recommendation that is followed by periodic inspection or it might be that the present consumer unit is in a bad condition. This blogpost discusses about some considerations at the time of replacing a consumer unit.
Important considerations and possible drawbacks
You need to discuss with your client to know they are fully aware of what has been involved. A consumer unit replacement might be a major task and if it is not planned properly, then it can leave the contractor with several problems.
Some accurate checks and necessary information may prevent various problems from taking place that help with design and installation of the newer unit. You need to follow these important considerations:
- For supply isolation, see if there is any switch disconnector on the supply side of the CU? If it isn’t, try to make necessary arrangements with the Distribution Network Operator or DNO
- Find out if different kinds of CU configuration are discussed with the client. They might be high integrity, RCBO set-up or dual RCD.
- Know the discrimination between any sub-mains to be considered.
- If the present installation has been assessed to ensure the CU replacement can be done.
Many installers fail to completely appreciate how many regulations are related to a consumer unit replacement in London. The mistake is to perform the work and find the existing defects on the Electrical Installation Certificate or EIC. The defects may be recorded in the Comments on the present installation section in case they do not result in a C1, C2 or FI classification code.
By abiding by BS 7671, the installers will be reminded of the requirement to comply with the Electricity at Work Regulations 1989. This will consist of the following:
- Regulation 12 Means to cut off supply and isolation
- Regulation 13 Precautions to work on the equipment that are already dead
- Regulation 14 Work on or near live conductors
Requirements of BS 7671
A consumer unit replacement is generally categorized as forming an alteration that falls into the category of new work. This should be erected, designed and then verified in line with BS 7671 as has been required by the Regulation 110.1.2 (vi). The safety of present installation should not be impaired with Regulation 610.4.
Regulation 132.16 states the needs of additions and alterations with the existing equipment along with distributor’s equipment that should be adequate for the alteration. The bonding as well as earthing should also be sufficient for the installation process.
The design should consider Regulation 415.1 for extra protection by 30mA RCD and necessary consideration to stay protected against unnecessary tripping of the RCD(s). This is usually needed to meet the requirements of Section 314, Division of Installation.
Account should be taken of the instructions of manufacturer, as needed by the Regulations 134.1.1 and 510.3.
On completion, the identification notices in line with Section 514 that should be fitted and an EIC must be issued to meet the needs of Sections 631 and 632, as stated in Regulation 631.1. Finally, the work has to be notified completely.
Best practice of the industry
The customer has to be encouraged for an Electrical Installation Condition Report or EICR to be done before replacing a consumer unit. This will enable the contractor to set up the requirements of Regulation 132.16 that enable the new consumer unit installation to occur. The suitability of present bonding and earthing arrangements need to be confirmed with the DNO’s equipment and meter tails that include – polarity of incoming supply.
If there are any defects or faults highlighted at this stage, then this will allow for remedial work to be planned and performed, as a part of consumer unit replacement or before its replacement. The contractor won’t be stuck at the end of the day with faults to clear that had not been built with the original contract.
The test results or EICR are there to help with design and choosing of protective devices for a new consumer unit. Proper care is needed when you choose protective devices to ensure maximum disconnection times are met and maximum fault current breaking capacity of the device is not surpassed. If you install these devices without testing, then you may find that they are not properly rated.
In case of single-phase supplies for almost 100A, a fault current value of 16kA is generally quoted by the DNOs. This is considered to be the value at the point where service cable is joined to the DNO network. Till the installation is close to the distribution transformer, this value will be significantly less than 16kA due to the length of service cable.
Domestic consumer units are usually supplied together with Type B circuit- RCBOs to BS EN 61009 standards and breakers to BS EN 60898 that have a rating of 6kA as suggested by the installers of Electric Works London.
Regulation 434.1 requires the probable fault current should be determined at every relevant point of the installation process. You need to know that a relevant point is every point in the installation where the switchgear has been installed.