Predicting start-up inrush currents on LED lighting equipment

March 24, 2017

 

Welcome to this month’s  Blog: Predicting start-up inrush currents on LED lighting equipment ,and selecting the correct breaker ratings +fittings per breaker.

 

As LED technology is still a relatively new subject matter to many installing electrical contractors – the UK lighting industry has heard of an increasing number of incidents, where incorrect onsite assumptions/decisions have been made regarding the number of LED fittings that should be wired to particular rated breakers [ through the use of rule of thumb/guesses which are based on experience with older florescent technologies]

 

Old magnetic fluorescent lighting circuits ,used to be much more predictable in terms of using a rule of thumb to estimate the required breaker ratings [ though still dependant on cable  length /site voltage /onsite temperature ]

 

Now, there are many electronic variants of driver technology  on the market -due to  the introduction of many differing types of LED driver technology – such as  SELV/non SELV – low power factor or intelligent  self-correcting high power factor drivers –wide and narrow voltage range drivers -also AC direct drivers-and driverless AC LEDs+ variants in surge suppressing/handling/and wattages/efficacy !

Even similar types of design from different manufacturers can have wildly varying inrush ratings.

 

Also bearing in mind that these differing designs ,will also differ in inrush  level/time ,depending on how they are implemented with regards to source/load matching and behaviour at low temperatures – or low/high  onsite voltage levels .

 

Any of the technologies mentioned above ,could also  have either a soft start function ,or suffer from voltage spikes on the LED array when starting up . The soft start function is where the voltage /current to the LEDs is slowly ramped up at start-up , this avoids damaging the LED – especially in cold start up conditions [ rise time of the LED current is often confused with AC input inrush time of the driver – due to translation issues when converting from Chinese to English ]

So – there are many potential variables!

 

To recap – using the old rule of thumb method on a multitude of differing technologies - from differing manufacturers ,without taking into account the cold start-up inrush behaviour of the particular product, can result in onsite issues – such as nuisance tripping/blackouts [ with onsite safety at risk ]

 

There is no common rule of thumb/assumption that can be applied to all LED products -to guarantee reliable onsite operation ,with regards to selecting the correct breaker type and number of luminaries per breaker.

 

Electricians only used to working with old fluorescent magnetic control gear ,could experience onsite problems- if they make the incorrect assumption that start-up current inrushes are lower on LED lighting equipment than on magnetic gear.

 

Why is this the case?

 

Bear in mind that  good quality LED lighting is flicker free – this means the LEDs are running from a good DC constant current source , this requires a large electrolytic capacitor to charge up first, and reduce DC ripple/flicker.

 

– old magnetic gear is AC driven [ they rely of the light decay time of the phosphor material in a T8 tube to eliminate flicker] as a result it does not need a large DC smoothing capacitors –and has a lower inrush current.

 

Please note ,there are can be many variables with this subject matter – cold start inrush current ratings my vary onsite [ as the cable length affects the  impedance between power source and load – the inrush levels and decay time will alter, and if the inrush time period is extended –it may exceed the breakers rated clearing time ,which results in onsite nuisance tripping .

 

In such a scenario ,the installer may find they have to re wire the site to reduce the number of fittings per breaker

 

We recommend, that when using any LED based products –that installing electricians always refer to the manufacturers/suppliers , informing them of the onsite circumstances -regarding make/model/wattage of product- and onsite worst case onsite temperature’s -and with some indication of the cable lengths used ,and  if the onsite supply is low [as it may extend inrush time and could nuisance trip the breakers]

 

It is worth noting that C type breakers have a longer clearing time ,and are more suitable for LED products – the clearing time allows the breaker to allow a high current inrush over a longer narrow time period  [ that will not overheat the wiring ,or nuisance trip on extended cold start inrushes – if rated correctly ] – but attention still has to paid, to the maximum allowable number of fittings per breaker as well.

 

So -to proceed without knowing  the particular product’s inrush current and breaker recommendations ,and to make incorrect rule of thumb assumptions, is to court onsite disaster.

Even if the site seems Ok ,the breakers could be pushed to their limit and eventually wear down and start nuisance tripping at a later date  - or when onsite supply voltage levels /temperatures vary.

 

We release this blog in the hope that it can help avert unnecessary onsite complications/costs and damage to reputation - and help installers understand the aspects of this relatively new and ever changing technology.

 

In the past lamps used simpler less efficient technologies [ using blown glass and electrodes ]– however ,these day we are dealing with far more complex electronic circuit designs and  many of them [ using semiconductor technologies] –in order to get the most performance /reliability and problem free -ease of installation ,individual  product knowledge is required .