Water Supply Industry Interpretations and Advice

  • Area of Interest : Backflow prevention
  • Topic : Type DC
  • Title : Use of Type DC arrangements in laboratories
  • Clause Reference : Regulation 4 (6) (c)
  • Reference Number : B41
  • Decision Date : May 2010 (revised March 2014 & January 2015)

Subject to the provisos below, Type DC arrangements installed on laboratory taps are a permitted means of point-of use fluid category 5 backflow protection in all laboratories.


  1. Users of such arrangements must be aware that the acceptability of such arrangements is dependent upon the outlet, including the outlet of any hose attached:-

      • remaining unrestricted which would mean that it could not be attached to any apparatus that would create a back pressure;
      • discharging at least 150mm below the air vents of the DC device, ruling out raising the hose outlet above this point; and
      • the spillover level of any receiving vessel is at least 150mm below the air vents

  2. Only arrangements which satisfy these requirements will be accepted. Where such arrangements are identified as being used inappropriately, the Water Supplier can require an alternative means of backflow protection at the point of use in those premises. 

  3. Type DC devices are permitted to be used on outlets in fume cupboards which are handling chemicals.

  4. Type DC devices are not suitable for outlets in biological safety cabinets or microbiological safety cabinets (biosafety cabinets) used with materials contaminated with pathogens which require a defined biosafety level. Water outlets in biosafety cabinets should be fed via a Type AA, AB or AD air gap. 

  5. Where the consequence of a backflow incident would be especially serious because of the nature of the substances handled in specific laboratories, a risk assessment should be made to decide whether zone or whole-site protection is required in addition to the point of use protection.