Please oh please – make functional safety easier

We all appreciate how modern and even not-so-modern technology makes life on the home front simpler and more flexible. There are plenty of wonderful shortcuts to save time and effort: the subway has an escalator, so you don’t need to take the stairs; your power drill is cordless; and your phone is no longer on the hall table but handily in your pocket. Well the good news is that functional safety is also getting simpler and more flexible. How? I’d like to share with you a nice new feature which will save you some hassles.

When you’re working with functional safety, you know you need to ensure a range of safety features are installed in your system and operating correctly. You also need to prove this by getting certification, and with good reason. These safety functions ensure your application offers protection against accidents for the staff involved in running it, and the tradespeople who maintain it.

Whizzed off their feet

For example, let’s consider that escalator. Before commissioning you need to obtain certification that the equipment meets legislative requirements. In Europe, this is the CE marking, mainly consisting of compliance to electrical safety (Low Voltage Directive = LVD), electromagnetically compatibility (EMC directive) and functional safety (Machinery Directive = MD)

So when it comes to functional safety, each escalator has to be checked and certified according to the machinery directive where the speed of an escalator is a safety-relevant factor, and overspeed protection is required in order to make the machine safe.

Safety requirements are specified in the Machinery Directive. Safe Maximum Speed (SMS) functionality makes sure the escalator never exceeds the safe maximum speed, ensuring that members of the public who ride the escalator are not whizzed off their feet. And we all know who will take the blame if they are – you.

So how do you obtain the SMS functionality? You limit the direct-on-line (DOL) electric motor driving the escalator, so it can’t run faster than the maximum safe speed. You perform a risk assessment and apply for certification to prove that the SMS function provides the level of protection it should.

Modern comforts, please

All that is fine, but these days there are modern technological finesses you can retrofit to the escalator to make it run far better. By connecting an AC drive such as the VLT® AutomationDrive FC 302 to the motor input, you can reduce electricity consumption, reduce wear, automatically collect operational data enabling you to predict maintenance schedules, and simply control the escalator so it gives your passengers a smoother ride. You want to enjoy these benefits.

When you install a drive into an escalator you undoubtedly get those benefits, but at the same time, the drive introduces a risk: the maximum speed is no longer fixed, but variable.

Mitigating the risk

So how do you obtain the SMS functionality, when the maximum speed is variable? You install a speed sensor with feedback to the electric motor driving the escalator. When the speed sensor detects that the escalator is reaching the maximum speed limit, it sends a signal back via the drive to the motor, to slow down. The speed sensor works fine until it wears out and of course, it will need replacing now and then. Anyway, you put in the legwork, perform another risk assessment and re-apply for SMS certification. When it arrives, you’re once again allowed to operate the escalator for members of the public.

But wait, isn’t the escalator also standing totally still, out of operation whilst I wait to receive the certificate again? – you ask.  And do I really have to go to all that effort, reapplying for the certification I already had? The answers are yes – and thankfully, no.

Here’s the simpler solution

This is where the nice new feature comes in. It’s in the form of two small options: the VLT® Safety Option MCB 151 and an extension to the MCB 151 called the VLT® Sensorless Safety MCB 159 option. When you include these options in your drive order, you are equipped to run SMS without the external speed sensor: it’s incorporated into the FC 302 drive and all you need to do is program the maximum speed into the drive. The MCB 159 option replaces the speed sensor you had installed on the escalator. You’re not introducing a new component and here’s the simplification: the SMS functionality is incorporated into the drive, which already has all the functional safety certifications you need.

With our integrated SMS solution, it is much easier to obtain certification (or re-certification), as overspeed can be completely taken out of the risk assessment for the machine.

Author: Stefan Mahne