Why DALI Must Be Wired With Double Insulated Mains

I am often asked why must the DALI communication bus (the DALI Line) be run in double insulated mains wiring?

Doing a search through available documentation it is difficult to find hard information to answer this specific question. The DALI AG (Action Group) http://www.dali-ag.org/ publish this  DALI manual.

Section 4.2, final paragraph, page 21 has the following statement:

“The isolation of the digital interface corresponds to the requirements of the base isolation. Tests are executed under the IEC 60 928 standard. Thus, SELV (Safety Extra Low Voltage) is not granted. The digital interface connecting leads can be wired jointly with the mains supply leads (e.g. 230 V), always provided that the isolation strength (2 x base isolation) will be observed.

Some DALI Control Gear manufacturers include wording in their installation document or data sheets like: ‘The network cable must be mains-rated’ or ‘The DALI signal is not SELV. The installation instructions for low voltage therefore apply’

Hardly Crystal Clear – So What Does This Really Mean?


Many control systems communication buses are run in SELV (Safety Extra Low Voltage) or PELV (Protected Extra Low Voltage) which require >3.5kV isolation from Mains (LV). DALI is different, although the nominal DALI Line voltage range is 9.5Vdc – 22.5Vdc it only has Basic Insolation (1500Vac) from mains.

IEC611240 defines this as FELV (Functional Extra Low Voltage) ie the nominal voltage falls within the ELV range but the isolation in insufficient to protect against a single fault. The DALI standard requires only Basic (ie 1500Vac) Isolation between the DALI Line and 240Vac Mains supply within the DALI Control gear, DALI Power Supplies etc

DALI-WiringWithin a DALI Fluorescent or LED etc Luminaire, the wiring between the Terminal Block and the control gear is typically run in single insulation wire and terminated in push in/punch down IDC terminal blocks.

In either of these cases a single fault could put 240Vac on to the DALI Line.

Even if some DALI manufacturers were to build Control Gear and Luminaires that did provide SELV isolation, DALI is an open standard and other manufacturers DALI units on that Line could compromise the SELV rating.

To protect against this, double insulated Mains (LV) wiring must be used

There Are Some Other Aspects To Consider

  • This does allow DALI Line wiring to run in the same bundle, tray, conduit as mains wiring without segregation
  • Before working on a DALI Line wiring, you should isolate all lighting circuit 240V supplies to all fittings connected to that DALI Line and to any DALI input and power supply units connected to that DALI line.
  • Any DALI input (ie switch or sensor etc) directly connected to a DALI Line must be mounted outside the clearance zones from taps and sinks etc required in Kitchens, Laundries and Bathrooms etc