Published:
5 March 2015
Category:
Q&A
Tim asks this question about immersion timer switches and the related MCB-protected immersion heater circuit. It has been answered by SELECT (Theme – Regulations and Legislation):
This Q & A is one of thousands posted in our Technical Expertise area, and answered on a daily basis by our Voltimum Experts.
Question: I notice that most immersion timer switches are rated at 13A or 16A. Whilst this is sufficient for the expected load of an immersion heater, my question is whether the timer switches themselves should be protected from overload past their rated values?
As many immersion circuits are protected with a 20A MCB, this means that the immersion timer switch is not protected against excessive current draw up to this point.
So my question is, is it OK to use 13A or 16A timers on a 20A MCB immersion circuit - assuming that the immersion load is within the timer's current carrying capacity?
Answer: Yes this is satisfactory.
The 20A circuit breaker is usually installed to provide overload, short circuit and earth fault protection for the cable, although on this particular type of load there is no likelihood of overload as the load is fixed e.g. a 3kW immersion heater.
Take the equation (see Regulation 433.1.1): design current (Ib) = power / voltage = 3000/230 = 13A
To protect the cables from overload, the following needs to be satisfied:
Ib should be less than or equal to In ( the nominal current of the circuit breaker) and Iz (the current carrying capacity of the cables should be equal to or greater than In.
In the situation you describe, overload protection is not required and so long as the current carrying capacity of the cables is greater than or equal to Ib (taking account of any corrections factors), the regulations are satisfied.
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