The best way to use our products is to know their characteristics, qualities and limits. So below are several Frequently Asked Questions that can help you in choosing your products.
For any general question, contact us at: firstname.lastname@example.org
1 - What is a positive opening switch?
Positive opening switches don't present any elastic connection between the mobile contacts and the actuator on which is applied the activating force. All the switches with positive opening of the contacts show the symbol of "positive opening" printed on the cover cap. A switch with a flexible bar on the head does not have the positive opening of the contacts.
When only one switch is employed in a safety function, it has to be activated with positive opeing. For safety appliances, you need to use the opening contact (normally closed) with "positive opening" : all the switches showing this symbol are provided with NC contacts with positive opening.
If there are two or more switches, it is advisable to get them to operate in opposite ways, for example:
- the first one with a normally closed contact (opening contact), activated from the guard with positive opening;
- the other one with a normally open contact (closing contact), activated from the guard with non-positive opening.
This is a common practice which does not exclude, whenever justified, the use of the two switches activated with positive opening (see diversification).
2a - What is a slow-action contact block?
The contacts' movement inside a slow-action-switch' armature is proportional to the activation speed (if the switch is activated slowly, the contacts open slowly). This kind of contact block is suitable for applications with low voltage and fast switch' activation movement (if the activating movement is too slow, the electric arc could persist between the contacts during the opening phase).
2b - What is a snap action contact block?
The contacts of a snap action switch move independently from the activation speed (even if it is activated slowly, the contacts open quickly. You can hear a "click"). This kind of switches is suitable for application involving high voltages and/or slow activating movements (the electric arc is broken by the click of the structure holding the contacts).
3 - How many switches do I have to install in my machine to be in line with the existing norms?
The risk category of a machine is determined by several points: seriousness of accidents that it might cause (death, deep injury, simple excoriation, etc.), presence of workers in dangerous zones (rare, frequent, permanent), possibility of prevent the accident (some machines for example make slow movements, so it is easier for a worker to avoid the danger). Please see general catalogue and Norms EN 1050. Once the risk category has been defined, the designer needs to project a safety circuit suitable for that category. Depending on the category we could associate: one switch, two switches or two switches connected to a safety switchboard.
For further information and practical examples, please see general catalogue. (please see Norm EN954/1)
4 - Is it true that the safety switch should be red?
The Safety Norms does not specify any rule for the color of the safety switches. Few manufacturers have chosen the red color with the purpose of distinguish safety buttons from their standard production, but any color can be chosen.
5 - I am looking for a switch opening a NC-contact when activated on the right side, and another NC-contact when activated on the left side.
Pizzato Elettrica designs many different types of independent- contacts switches with B16 contact block and a head with turning lever. The most frequently adopted switches are FR 1631, FR 1654, FD 1631, FP 1635, etc.
6 - Is it true that from year 2001 all the switches should have metric threading for the cable glands?
It is not true. From March 2001 indeed, a new rule EN50262 has came in force, that calls for the adoption of metric thread in new projects . This however, has no influence on old projects. Pizzato Elettrica will continue manufacturing switches with PG thread, but versions with metric thread are already available on request.
7 - Can I use a switch with revolving lever working only in a unidirectional way?
Yes, is possible with the switches of the series FD, FP, PL, FC since they have a special piston that can work on one-way only. For further information and practical examples, please see general catalog.
8 - What kind of switch do I need to use in case of a safety application?
For the installation of switches with safety functions use only switches with positive opening of the contacts; connect the safety circuit to the normally closed contacts (11-22, 21-22, 31-32); activate the switch at least up to the positive opening travel shown in the travel diagrams and set the switch in such a way that, by opening the protection guard, it completes the positive opening.
See Technical data and utilization requirements
9 - How could we define the "differential travel of a contact block"?
In snap-action contact blocks the differential travel is the difference between the point of snap in forward travel and the point of snap in backward travel. In other words, the contact block does not snap in the same point. For example, in the contact block B5 the point of snap in forward travel is 2mm, while in the back travel is 1 mm. Therefore, his differential travel is 1mm. Note: in slow-action contact blocks there is no differential travel.
See Technical data and utilization requirements
10 – What is the MTTFd of a safety switch?
When dealing with electromechanical devices, there is no point in talking about MTTFd without making reference to the utilization frequency of the device. While it is assumed that the deterioration of electronic devices only depends on time, this can't be considered true for electromechanical devices. To calculate the MTTFd, you need to know another parameter - B10d - indicating the number of operations at which 10% of the samples tested becomes dangerously damaged. The two parameters are tied to the formula MTTFd=B10d/0.1*nop, where nop stands for the number of operations expected over one year.
See Introduction to Safety
11 – Which Performance Level (PL) can be reached with a safety switch?
The PL of a safety circuit depends on the structure of a circuit, on the reliability of the devices making up the circuit and is also linked to some other parameters. This is why it does not make any sense to ask whether a particular safety switch is suitable for a circuit with a PLe value, since the top PL level that can be reached depends not only on the device itself, but also on many other parameters, such as the circuit's structure, the diagnostic coverage, the causes of common failure, the application frequency etc. Therefore we can say that a safety switch can be used in circuits up to a PLe value, but the reaching of this PL level depends on other parameters.
See Introduction to Safety
12 – What parameters do I need to know in order to calculate the Performance Level (PL) of a safety circuit? Where can I find them?
We need to consider 4 parameters:
- Safety Category of the circuit (one needs to know the circuit architecture, i.e. if we are talking about a single-channel circuit, a single-channel circuit with cyclic test or a double-channel circuit);
- MTTFd of the various devices used (this parameter indicates the reliability of a particular device);
- Diagnostic Coverage (DC, this parameter indicates how far the system is able to “self-monitor” any of its own malfunctions);
- Common Cause Failure (CCF, only applied to circuits of categories 2, 3 and 4, assessing all the common cause failures which may impair the system redundancy).